303 brit in a .308 barrel?

[nekshot]

I am planning my work for the winter and my next project is a 95 mauser with a barrel (was a 30-06 cut at chamber leaving .455 to start with). I thought I would chamber this one for 300 savage but that cartridge looks boring to me in my present emotional state. I have plenty of 303 reloading goods and I would love that cartridge in a .308 bore. I know this whole deal is screaming 30-40 but I don't have those dies. Any one do the 303 into a .308 bore?

[Ballistics in Scotland]

My guess is that you wouldn't get dangerous pressure for a Mauser by firing a .312in. sheet jacketed bullet. But my guess probably isn't good enough for you, and certainly isn't for me. Besides, the rifle may someday be in the hands of someone who will use solid base boat-tails or brass or copper solids. Who knows what generations yet unborn will have to use for bullets?

But you should try your dies with .308 bullets. You will have to either change the expander button or reduce it with abrasive paper in an electric drill. But I think most commercial dies will reduce the neck enough for .308. This is not the route to a benchrest rifle, but for hunting or informal self-expression the slight extra neck expansion on firing is unlikely to affect accuracy or brass life enough to worry about. I have seen this work very well with a Martini action and M1919 Browning machine-gun barrel.

.300 Savage is unlikely to be as available or cheap as .303 nowadays, but does offer one advantage. It would probably be easier to get it to feed through the Mauser action. If I was set on the .303 I would probably try to adapt a Lee-Enfield magazine.

[HollowPoint]

Yes Sir:

My very first re-barreling attempt was with my Lee Enfield No.4 MkI. Since mine was a budget project and 303 barrels were way more expensive than 308 barrels I opted to chamber mine with the 303 chamber reamer necked down to the 308 bore.

The benefits are many. I to already had the brass, dies and experience with reloading the 303. I also had a few 30 caliber bullet molds so I didn't have to buy any more molds to address the slightly larger diameter of the 303 bore.

I've tried to find a lighter weight cast bullet that will shoot well in my Enfield but the best shooters are the heavier cast bullets. Mine is a dedicated cast bullet shooter. It will shoot the 100 grain jacketed bullets real well but for some reason it will only shoot the lead-heavies with the same accuracy.

I should mention also that this same enfield has been somewhat bubbafied. I shorted the barrel and stock and it's muzzle-threaded to accept a suppressor. Right now I'm experimenting with powder coated bullets through it. I already know that the plain lubed and checked bullets will shoot accurately but I'm trying to squeeze a little more velocity out of it while maintaining accuracy.

HollowPoint

[Outpost75]

In the 1970s LTC John George, author of "Shots Fired In Anger" was on the NRA Board. He lived in DC and was a regular around HQ. A real rifle nut in every sense. When I was on active duty I had I had worked with him on a project in which surviving CBI vets were interviewed to glean bits of field and tradecraft related to jungle warfare which might have otherwise been lost. John spent many of his postwar years in East Africa until the late 1960s and had numerous contacts among Safari Club International and Brits he had served with in the CBI. I traveled to UK to meet with some of those old soldiers. One of the gents whom I met was a retired British Army cartographer who was an avid big game hunter who had been an adviser to Wingate's Chindits. Most of his African postwar "expeditions" were under the auspices of the British Museum, though I realize now that Mi6 could have been involved in some of it. The old boy had a "bush rifle" put together by Holland & Holland, which was essentially a scoped, heavy barreled jungle carbine in 7.62mm NATO. That rifle appeared in the British magazine Shooting Times and Country. I carried that dog-eared copy around for years in search of a gunsmith to build my "fantasy No.4," and while in those days I was thinking hard towards 7.62 NATO, I would later be convinced otherwise and eventually went .303.

When I worked for Ruger in the 1980s the company had a project making M77 sniper 7.62 rifles for the RCMP to replace their converted 7.62mm No.4s they were having troubles with, as to durability. They regretted ever getting rid of their .303s, but the .303 surplus ammo was mostly gone and they needed a 7.62 rifle because they drew ammo from Cdn. Forces stores. I spoke with RCMP and Cdn. Army gunsmiths at GREAT length. I was told that there were always "issues" of one sort or another trying to make a reliable 7.62 rifle on the No.4. While the Long Branch and Savage receivers had better steel than UK production and were strong enough, feeding was never 100%. When shot "alot" headspace loosened and they were always refitting the next larger bolt head "before the barrel was shot out." I was advised in strongest possible terms to forget about 7.62, but to keep the rifle in its original .303 caliber, that I would be happier.

I was told it was completely OK to go ahead and use a match quality .308 groove diameter barrel, of 10-inch twist, to enable use of common bullets and adequately stabilize everything. And that you could shoot ordinary .303 ammo down the .30 caliber barrel and this was just fine! I was skeptical at first of that claim, but Ruger had an order for 1000 No.3 single shots in .303 British for commercial sale in Canada. So, I had sufficient engineering justification to build pressure barrels with both .303 Brit and US cal. .30 rifling dimensions, because we sure as hell weren't going to tool up to make .303 barrels just for a 1000 gun order. Radial copper pressure test barrels were ordered from CIL's sub-contractor, differing only in bore and groove dimensions, one being to SAAMI dimensions for the .303 British and the other of 4-groove, Government form as used for the .30'06. I tested them with everything, Cdn and UK military, US and Cdn. commercial and my handloads with Sierra and Hornady bullets of both diameters. Bottom line, OK, yes, pressure is higher, but no, it is not dangerous. The resulting sample means are well within design limits of a sound Long Branch, essentially +3500 psi or 48,000 max vs. 44,500 max. - much less than 7.62 NATO at 52,000 which gave problems with bolts compressing, receivers stretching and spotty feeding.

On one of my Canadian visits I was offered a DCRA style No. 4 Long Branch target rifle built in the 1970s with "shot out" barrel. I got it imported on a Form 6 OK and had scope mounts made in the experimental shops at Ruger. I removed the fragile A.J. Parker target sight, rebarreled it and installed a WW2 style battle sight, zeroing the two apertures at 200 and 400 yards with 180-gr. softpoint ammo. I can use iron sights or scope at will and stripper clip load from the top. The scope comes off or goes back on easily without loss of zero. The barrel is a hammer-forged 7.62 blank of 4-groove government form, chamber cut with the .303 British SAAMI pressure-velocity test barrel reamer. This is a minimum chamber which does not blow the shoulder forward like the wartime "trench" chambers do. It shoots any factory .303 Brit ammo fine, or handloads with common .308 diameter bullets. Brass life is good. A solid 2 moa rifle with good lots of ball ammo, and 1.5 moa or better with handloaded Sierras.

Short-stroke bolt and 10-shot magazine give it good rate of fire. I have shot it for laughs in Modified Infantry Trophy at Cherry Ridge, NJ starting with full magazine I can bang those off, reload with two strippers and usually get the second ten off in 50 secs. at the reduced, simulated 500 and 600 fired at 300 yards with enough hits on the "E" silhouette to get my bonus points. I score about the same with it as I do my M1 Garand, though I may get off a few more shots with the M1, number of hits per string is the same until you get up to 300, where the Garand has the advantage sitting rapid not having to work the bolt.

182215182217182218182219

[longbow]

Not sure I am understanding completely. Are you saying reload .303 Brit with .303 boolits (0.312" to 0.315") or just chambering with .303 Brit reamer but loading 0.310" boolits?

Either way I guess as BIS points out there is a possibility of someone sometime down the road chambering a standard .303 British round with 0.311"/0.312" solid bullet which might be a bit much. That is if it will chamber.

Lead boolits shouldn't be an issue pressure wise is my guess. But just my guess.

If you could shorten the chamber just enough that a standard .303 Brit round wouldn't chamber then there would be no worry that way and you would just push shoulders back a bit on you brass but then that would take modifying dies some.

HollowPoint:

What lightweight boolits have you tried in yours?

I am shooting the Mihec 316410 130 gr. version in my Lee Enfields and they shoot very well for me. That same mould is available in 0.311" as well.

I did try 100 gr. PP boolits but got terrible accuracy. All holes in targets were round so not tumbling but poor accuracy. I figured it was the long jump to the rifling with that short boolit that was the culprit.

I will agree that I have had better accuracy with the normal 200 to 220 gr. boolits though but not bad at all with those 130 gr.

Longbow

[longbow]

Well I guess Outpost75's post answers the pressure issue ~ not a problem.

[pietro]

.

Getting rimmed cartridges (.303 Brit, ,30-40, etc) to feed through an action designed for rimless cartridges is a non-starter for me - and I'm less than interested in a single-shot boltgun.

YMMV, of course.


.


.

[NoAngel]

303 british with a .308 bore.......Didn't we call that a 30-40 Krag? LOL!!


IT'd be no different than any other wildcat. Care taken, it'd be just fine. It would certainly open up lots of doors for bullet selection.

[nekshot]

what I have trouble figuring out is how to run a 303 reamer into the barrel and have a neck space for a .308 projectile. That special reamer spoken of would possibly solve my question but who would have one of those? Concerning the feeding this action is clip fed so feeding is no issue. I must be honest to my self and state the long neck for supporting and enclosing the cast boolit takes my fancy. Maybe I will just ream it 300 sav and open the throat alittle to accept the cartridge length(that is taking a 308 brass and running it up a 300 sav die and don't cut off extra neck length). That gives a nice long neck which is what I want. Or the 30-40krag would take care of all I want but would require some mullah layed out and I don't have any to spare right now! This project is really playing with my head!!

[Outpost75]

I used standard .303 reamer, cases and dies. Use a pin retainer on decapping rod with no expander ball, so cases hold .308 jacketed bullets snugly. Neck clearance is not an issue. Use Lyman M die to expand necks for .312" cast bullets or jacketed Hornady .312" softpoints with 40 grs. of IMR4064. For cast I use NOE #311299 clone with 30 grs. of RL7.

[HollowPoint]

I have an NOE Spitzer bullet mold that drops bullets at just a little under 200 grains that shoots like gangbusters out of my Enfield. I also have a Lyman 311296 that was hollow pointed by Eric over at HollowPointService.com. I think that's the right number for that mold. I'm just guessing right now. It drops bullets at around 180 grains. I also have been trying to get my other Lyman 30 caliber mold bullets to work but for some reason they don't shoot as consistently accurate out of my guns. It's listed as a 115 grain bullet mold but mine weigh more like 130 grains. I hollow pointed one of the cavities in an effort to bring the weight down but it's on a couple grains lighter.

They accurate enough for coyotes out to 150 yards but beyond that distance I don't want to risk it; especially when I can shoot jacketed out of it that are alot more consistent.

The hollow pointed Lyman 311296 mold is my go to bullet mold for this rifle. It also shot real well in my K31 when that rifle was chambered in a 30 caliber bullet.

HollowPoint

[Multigunner]

IIRC when Ruger built a custom .303 chambered No.1 rifle with .308 bore, special odered for a gentleman who wished to test all the best target grade .308 bullets and possibly have the best manufactured in .311-.312, they found a standard .303 British cartridge with .311 bullet fired in a .308 bore increased chamber pressure by 7% around 3,000+ PSI.
I found that information posted on a long ago board by a former Ruger Employee.

It was perhaps six years later that Ruger built a few No.1 rifles on a commercial basis in .303. I don't know what bore size they used, but UK shooters at the time preferred undersized bores for long range target rifles.
An undersized bore apparently prevents blow by when using boat tail bullets, though that's not normally a problem if bullet and bore are well matched some European ammo has been found to have undersized bullets especially in .303 and .308 ammunition.
Erosion of the throat and first few inches of the bore can cause blowby and a bullet a few thousandths over the standard diameter can cure that.
The early FN FAL rifles had slightly undersized bores to increase bore life, up to 50,000 rds plus in some tests.

[Outpost75]

I also can say from personal experience, measured pressures from sloppy wartime "trench" chamber are higher than SAAMI chamber with tighter .308 groove, because oversized throat of .316-.318" enables bullet base to be upset so that it must be extruded down again during initial shot-start, whereas sporting chamber supports bullet base and prevents excessive base upset, so that extrusion effect of .312 bullet into a .308 barrel spikes pressure less than "nail-head" effect of trench chamber.

[HollowPoint]

what I have trouble figuring out is how to run a 303 reamer into the barrel and have a neck space for a .308 projectile. That special reamer spoken of would possibly solve my question but who would have one of those? Concerning the feeding this action is clip fed so feeding is no issue. I must be honest to my self and state the long neck for supporting and enclosing the cast boolit takes my fancy. Maybe I will just ream it 300 sav and open the throat alittle to accept the cartridge length(that is taking a 308 brass and running it up a 300 sav die and don't cut off extra neck length). That gives a nice long neck which is what I want. Or the 30-40krag would take care of all I want but would require some mullah layed out and I don't have any to spare right now! This project is really playing with my head!!

All I did when I bought my 303 reamer was slowly turn down the neck and throat sections of the reamer on my metal lathe using the finest emery cloth that would remove material from the hardened metal of the reamer. It wasn't the best way to go about it and it took alot of time cause I deliberately went slow so that I could take measurements every few seconds or so but I got it down to the dimensions I wanted. Now I'm the proud owner of a decent shooting 30/303 Lee Enfield rifle.

One of the things that really concerned me by doing it this way was being able to keep the flutes where I had turned the reamer down as sharp as they were when I first took it out of the packaging. I don't think it ended up being as razor sharp as it was at the neck and throat section as it could have been but it was sharp enough to allow me to ream a nice clean chamber.

As a precaution I initially stair-stepped my chamber cuts using various drill bits before I commenced with the chamber reamer. Since I turned it down on the front end, I didn't want to subject it to any further dulling by using it to cut the full length of the chamber. If I had to do it again on the same budget, I'd most likely do it the same way but, if I did it with more funds available to me I'd opt for a custom reamer and forgo the stair-step pre-drilling.

HollowPoint

[samari46]

Outpost75, nice looking enfield, kinda sorta reminds me of the L39A1 in 7.62 nato with the hammer forged barrel. Which I had and stupidly sold for what I cannot remember. Thanks for posting the pics. Only one question, why is the scope so far offset to the left?. Thanks Frank

[EDG]

I think you have something wrong.
A .303 bullet measures .311 to .312. By definition the chamber is large enough for a .308 projectile.

To get a .303 reamer to work the pilot must fit a .300 land diameter. A .303 reamer with interchangeable pilot bushing would work fine.


what I have trouble figuring out is how to run a 303 reamer into the barrel and have a neck space for a .308 projectile. That special reamer spoken of would possibly solve my question but who would have one of those? Concerning the feeding this action is clip fed so feeding is no issue. I must be honest to my self and state the long neck for supporting and enclosing the cast boolit takes my fancy. Maybe I will just ream it 300 sav and open the throat alittle to accept the cartridge length(that is taking a 308 brass and running it up a 300 sav die and don't cut off extra neck length). That gives a nice long neck which is what I want. Or the 30-40krag would take care of all I want but would require some mullah layed out and I don't have any to spare right now! This project is really playing with my head!!

[EDG]

Commonwealth shooters may eventually have to bite the bullet and switch over to .308 bores for their .303s.
Well you say just keep on using .303 barrels?

If you go lurk at Lee-Enfield forums you will learn that the original Lee-Enfield barrels do not perform well with modern boat tailed bullets. The commonwealth shooters claim that only the Mk VII bullet will work right. This is a long pointed flat base bullet that upsets to fill the oversize bores better than any boat tail. However there are no current manufacturers of this bullet type. It was only produced in military ammo plants. Without this bullet the original Lee Enfield barrels are a handicap since a large percentage are oversize.

I think a well designed chamber using a .308 blank will be a much better alternative for .303 shooters. They would have a huge choice of state of the art bullets suitable for a .308 barrel.

[Ballistics in Scotland]

Well I guess Outpost75's post answers the pressure issue ~ not a problem.

Indeed it does, in a very detailed and authoritative way, and is just what I would have expected - for the types of ammunition named, and the "normal bullets" mentioned. But there is always the chance that someone will find an unsafe bullet type. It could conjoin with unsafe loading practices by some future owner. At the very least the rifle should be marked with the correct bullet diameter. I believe Ruger also chambered the 7.62x39 with .308 groove blanks, and made similar denials, probably as justified, when people questioned it.

There is very limited benefit in slightly shortening the chamber to prevent insertion of a standard .303 round. It is quite easy to grind a little off the bottom of the sizing die. I've done all this to load .255 Jeffery with a .25-20 reamer, dies and cases. Or grinding a little off the top of the shellholder would probably be enough, though that can't be the tenth of an inch I needed.

For a short chamber with conventional reamer you would have to cut the rim recess with something besides an ordinary chamber reamer. But that isn't an insuperable obstacle, as you don't need the extreme degree of concentricity which neck-to-body alignment requires. A reamer with interchangeable pilots is indeed the best, and essential if it belongs to a gunsmith or you rent it. But it is possible to remove a few thousandths from a solid pilot It can more easily be kept a true cylinder with a fine oilstone in the lathe. I wouldn't do it in a high-speed electric drill, which tends to vibrate and make things go oval. If I didn't have a lathe I would use a slower electric screwdriver, with something supporting the centre hole in the pilot end of the reamer.

Many No4 Lee-Enfields, more than the earlier rifles, have oversized bores, and I think it is this in particular that makes the flat-based bullet advantageous. I know of people getting good results with .318 bullets made for the J-bore 8x57. Some very good heavy target barrels were made by Parker-Hale etc. in the latter days of .303 target shooting which held the official .303 dimensions more closely than most military barrels. I wouldn't be surprised if somebody in the US is sitting on a large but unpublicised stock of these.

[nekshot]

Ok, I see I need to talk to the reamer rentals and I believe I can get this to work. Thanks to all for input especially outpost 75. One thing I left out in my mind is I really want to screw around with BP in the 303 and be able to use my boolits for the .308 bore. So I plod on !

[Outpost75]

Big issue if you want to fool with BP is having a bullet with enough lubricant capacity to keep the fouling soft. Most .30 cal. designs don't hold enough lube. This is a bullet I use for plain-based reduced loads in a variety of .30 cals. and lubed with SPG it gave good results with BP in a .30-30

182281

[Ballistics in Scotland]

I think you have something wrong.
A .303 bullet measures .311 to .312. By definition the chamber is large enough for a .308 projectile.

To get a .303 reamer to work the pilot must fit a .300 land diameter. A .303 reamer with interchangeable pilot bushing would work fine.

The other issue is whether .303 dies will reduce the case neck enough to grip the bullet. I would prefer to reduce it a little undersize, even if it is only a thousandth or so, and use an expander button. If you simply rely on the die alone and insert the bullet, a trivial difference in loaded neck OD can be translated into a non-trivial difference in its grip on the bullet. I think most commercial dies will do this, but it is something to watch out for.

One of the big myths we often hear about the .303 is that it was originally a black powder round. In fact it was used as a stopgap for a brief period, in the form of a single pierced pellet loaded before the case was necked, a thing obviously unsuited for the recreational shooter today. They knew perfectly well from the start that a smokeless powder would be used, and even calibrated the sights of the Lee-Metford for a luckily-guessed fifty or so more ft,/sec. than the black powder would provide.

So it isn't designed the way a black powder cartridge would be. It is true that the amount of lube is a problem, and the short neck more or less prevents the use of a card and wax "cookie" behind the bullet, as was often used in longer necked rounds. It is possible to make a bullet with a single very large lube groove, which works well in low pressure cartridges such as the 8ga and 4ga big game rifles. But I don't like the idea for this one. In the original load, even with that slow-burning pellet, the peak pressure wasn't significantly lower than with smokeless, and there is a strong likelihood that the bullet base would cant, or be irregularly deformed into that large groove. If I wanted to use black powder in a .303, I believe I would paper-patch the bullet.

[EDG]

This is not a problem. I have a set of .001" increment pin gauges that I use to check the necks of my loading dies. I have several sets of RCBS .303 dies and the neck of the FL sizer is the exact same size as the neck of .308 Win size dies. All that happens is the .303 expander expands the case more than the .308 expander.
It seems that when they make .30 cal, .303 and 7.65 Mauser dies they use the same dimension for the neck in all of them.



The other issue is whether .303 dies will reduce the case neck enough to grip the bullet. I would prefer to reduce it a little undersize, even if it is only a thousandth or so, and use an expander button. If you simply rely on the die alone and insert the bullet, a trivial difference in loaded neck OD can be translated into a non-trivial difference in its grip on the bullet. I think most commercial dies will do this, but it is something to watch out for

[Ballistics in Scotland]

This is not a problem. I have a set of .001" increment pin gauges that I use to check the necks of my loading dies. I have several sets of RCBS .303 dies and the neck of the FL sizer is the exact same size as the neck of .308 Win size dies. All that happens is the .303 expander expands the case more than the .308 expander.
It seems that when they make .30 cal, .303 and 7.65 Mauser dies they use the same dimension for the neck in all of them.

That is about what I would have expected, and I also know of someone finding Lee dies satisfactory that way. Indeed I did, when I loaded some cases with his dies for experimentation with .308 bullets in a .303 P14 Enfield I had bought to rebarrel in .300 H&H. Those dies were made a long time back, but it is unlikely that they have changed.

Incidentally I got good accuracy, for a frosted bore, with lightly jacketed sporting bullets (though more testing might have revealed the odd rogue shot, and hunting would then have revealed it at the worst possible moment.) Nosler solid based boat-tails, excellent bullets when not misused, almost all tumbled, with one-sided engraving to show that centrifugal force had pressed them very hard against one side of the bore. Just the occasional one behaved impeccably. I had a flat piece of steel with .004in. deep grooves from playing around with acid etching, and it took quite a bit of force in an engineer's vice to make a bullet engrave as precisely as centrifugal force had done.

[DrLiberty]

I am planning my work for the winter and my next project is a 95 mauser with a barrel (was a 30-06 cut at chamber leaving .455 to start with). I thought I would chamber this one for 300 savage but that cartridge looks boring to me in my present emotional state. I have plenty of 303 reloading goods and I would love that cartridge in a .308 bore. I know this whole deal is screaming 30-40 but I don't have those dies. Any one do the 303 into a .308 bore?

Yes, I have and with disastrous results. While some .308 barrels MIGHT be ok with the.311 diameter slug, you run two major risks loading the larger .303 into the .308 case. 1: the slug will lodge in the barrel, causing excessive back pressure to the bolt, turning g it into a mini grenade. 2: the case will separate due to the slug lodged in the barrel, and the next round can fire out of battery, again creating a mini grenade right near your face / hands. I learned this the hard way.
Now, I can say that my Savage 99 WiIl fire this configuration, although it Chambers hard. Conversely, I turned a HK 91 into a mini grenade with this configuration.
Why? Well, ignorance and thrift. In the mid Obama years, slugs, powder, etc are expensive, if not impossible to find. I had some .303 rounds, which I disassembled and used the slugs to build .308.
At the time I was unaware that the Brits measured ".30 Cal" from the groove, not the lands.
Keep the .311 slugs for the .303 and .30-40's...

[Outpost75]

Regarding the left-offset scope mount in my No.4 bush rifle, this is to permit top reloading with chargers and alternate use of either the battlesight or scope. I have a Sako M28/30 Civil Guards rifle set up the same way.

[samari46]

Outpost75, thanks for the answer. Somewhere around here I have a bunch(never counted them) MKVII British bullets. Years back bought a large lot of WWII and later 303 British ammo and even though it was in the unopened 48 round boxes it had a less than ideal storage life. Gave up trying to clean off the corrosion and some cases actually had pinprick holes in them. So saved the bullets, destroyed the cordite and scrapped the brass. Frank

[john.k]

Ive fired thousands of 303 Mk 7 military bullets in a number of 30-06s and .Ive also used 303 military rounds in a Win 95 in 30/40 Krag,by simply setting the shoulder back 1/8" .No signs of any pressure issues.And from what I have read,a tight chamber and neck will boost pressure far more than a slightly oversize bullet.Ive also reloaded a 30-06 with cordite as well as 303 bullets.

[Outpost75]

Yes, I have and with disastrous results. While some .308 barrels MIGHT be ok with the.311 diameter slug, you run two major risks loading the larger .303 into the .308 case. 1: the slug will lodge in the barrel, causing excessive back pressure to the bolt, turning g it into a mini grenade. 2: the case will separate due to the slug lodged in the barrel, and the next round can fire out of battery, again creating a mini grenade right near your face / hands. I learned this the hard way.
Now, I can say that my Savage 99 WiIl fire this configuration, although it Chambers hard. Conversely, I turned a HK 91 into a mini grenade with this configuration.
Why? Well, ignorance and thrift. In the mid Obama years, slugs, powder, etc are expensive, if not impossible to find. I had some .303 rounds, which I disassembled and used the slugs to build .308.
At the time I was unaware that the Brits measured ".30 Cal" from the groove, not the lands.
Keep the .311 slugs for the .303 and .30-40's...

What you are describing above is the result of a chamber with a lack of neck-release clearance, not being a factor of barrel groove diameter. Firing cal. .30 M72 or 7.62mm M118 Match ammo in a TIGHT-NECKED target chamber will do the same thing. As long as the chamber NECK diameter had adequate release clearance for the larger bullet there is no hazard in a strong-actioned rifle which supports the case well.

When the German Army modified older J-bore 7.9mm rifles to use the larger bullet diameter S-bore ammunition, earlier '98 Mauser rifles had the chamber necks enlarged, and the throats reamed to a more gradual angle, to ease transition of the .325" diameter bullet into the .320" groove diameter barrel. While Type-S ammunition did exceed safe pressure in the Modell '88 Commission rifles so-altered, the stronger '98 actions tolerated the change with no issues.

[Texas by God]

And that is why 8mmMauser U.S. commercial ammo is loaded light with a soft core .321" 170 gr bullet. In case it finds its way into an 88 Commission rifle.

Sent from my SAMSUNG-SM-G930A using Tapatalk

[vzerone]

Yes, I have and with disastrous results. While some .308 barrels MIGHT be ok with the.311 diameter slug, you run two major risks loading the larger .303 into the .308 case. 1: the slug will lodge in the barrel, causing excessive back pressure to the bolt, turning g it into a mini grenade. 2: the case will separate due to the slug lodged in the barrel, and the next round can fire out of battery, again creating a mini grenade right near your face / hands. I learned this the hard way.
Now, I can say that my Savage 99 WiIl fire this configuration, although it Chambers hard. Conversely, I turned a HK 91 into a mini grenade with this configuration.
Why? Well, ignorance and thrift. In the mid Obama years, slugs, powder, etc are expensive, if not impossible to find. I had some .303 rounds, which I disassembled and used the slugs to build .308.
At the time I was unaware that the Brits measured ".30 Cal" from the groove, not the lands.
Keep the .311 slugs for the .303 and .30-40's...

The metric designation for the 303 British is 7.7x56R. 7.7 to inches is .30315 so that means it's measured on top the lands. You know the groove to groove is lot deeper then .30315. .308 does have a top of land measurement of .300. The groove to groove would be .308. The 30-40 Krag land to land is suppose to be .300 and the groove to groove is .308, but do to manufacturing tolerances and worn bores you'll find the groove diameters larger. I have one and my measurement is .3095. One should really shoot the correct diameter bullets in their rifle, although you can get away with a fatter bullets sometimes.