Reloading .303 British with cast bullets

[miker19]

I am new to reloading. I have some cast .303 bullets I want to reload but I'm not sure how much powder, what type powder, do I use fillers ect. Any information will help me. Thank you! The bullets are 160 grain and they have gas checks crimped on.

[Scharfschuetze]

I've not shot any cast boolit lighter than 200 grains in my Enfields, so I can't really recommend a powder charge.

The velocity level that you want to achieve will probably help you decide on what powder to use. Generally the fast pistol powders do well at the lower end, while powders like 2400, SC 4759 and IMR or H 4198 do well in the mid to high range velocities with cast boolits.

Share with us some of your details like:

Bore diameter (Enfields can vary quite a bit and that will dictate your optimal size for the boolit)
Boolit diameter (I generally size to .313" in mine, but I have two that take .315" diameter boolits)
Powders on hand to choose from
Performance level you wish to achieve

By the way, welcome to the forum. Which model Enfield do you have?

[1johnlb]

My suggestion is read all you can, this site, Lee reloading manual, Lymans cast manual #4. Before you ever load anything!


Sorry Miker19, lost my manners, Welcome to the board!

[PAT303]

18-20grns of H4227 is a good start. Pat

[longbow]

+1 on johnlb's comment "...read all you can...".

Do not start reloading without some knowledge. There will be lots of advice here but you should also get a Lyman Cast Bullet Handbook or the Lee cast bullet manual... or better yet both!

Also, there is information on cast bullet reloading here:

http://castpics.net/dpl/index.php/reloaders-reference/cast-bullet-data-lookup
http://castpics.net/dpl/index.php/reloaders-reference/reloading-manuals/190-ideal-handbook-38
http://www.303british.com/id37.html
http://castpics.net/dpl/index.php/the-archives/nra-reprints/170-cast-bullets
http://www.castpics.net/subsite2/ClassicWorks/CastBullets-s.pdf
http://www.castpics.net/subsite2/ClassicWorks/NRA%20Cast%20Sup1.pdf
http://www.castpics.net/subsite2/ClassicWorks/NRA_IRH.pdf

Some powders in the old Ideal handbook are obsolete but some are still available.

There tends to be a lack of load data for .303 British though there is enough around to get by with. you can also use .30-40 Krag data as the Krag operates at slightly lower pressure in the same size cartridge as .303 British... or close enough anyway.

I tend to use IMR4227 with cast boolits though I have tried several other powders as well: Unique, IMR4895, IMR4198, IMR43420, IMR4064.

My go to powder for general shooting is IMR4227 with boolits of 180 to 215 grs. as PAT303 says 18 to 20 grs. is a good start. IMR4227 and H4227 are pretty close.

With my 130 gr. boolits I use faster powders with good results but again IMR4227 works well too.

I also use fillers but do not just jump in to the world of fillers. I like them but many do not and they are a controversial sbject. Some claim to have bad experiences using fillers so it is best to avoid them and use powders that do not need fillers, at least to get started.

To start with you should slug your bore or better throat. Lee Enfields tend to have large groove diameters. Mine all run around 0.314" groove diameter and I had bad leading with standard .303 boolits that cast at 0.312" or so. I now have several "fat" moulds that cast at 0.316" so I can size to 0.315" when I install gas checks.

You should mic the boolits you have as well to ensure they are a good fit to the bore. If undersize they will shoot but accuracy will be poor and you will most likely get leading.

I hope that helps get you going.

Longbow

[Bad Ass Wallace]

I did a lot of testing earlier this year with a variety of 303's including Martini Enfields, LE's. SMLE's and P14's. The best and most accurate loads were with 3031 and no filler - 28.5gns for 1690fps and 29.5gns for 1804fps. The moulds I use come from CBE and are sized and lubed to .314".

BTW, unless you have some rather serious metrology equipment, you can't slug and measure a 5 groove Enfield barrel accurately.

http://img.photobucket.com/albums/v152/BAWallace/DC303_zpsb890b37f.jpg (http://smg.photobucket.com/user/BAWallace/media/DC303_zpsb890b37f.jpg.html)


These are some 10 shot bench rested 100yd groups
http://img.photobucket.com/albums/v152/BAWallace/303Martini220CBE105_zps1440956d.jpg (http://smg.photobucket.com/user/BAWallace/media/303Martini220CBE105_zps1440956d.jpg.html)

http://img.photobucket.com/albums/v152/BAWallace/Enfield303220CBE105_zps876d20bb.jpg (http://smg.photobucket.com/user/BAWallace/media/Enfield303220CBE105_zps876d20bb.jpg.html)

[longbow]

You can slug the throat though and that is the important bit. Mine are all about 0.315". That is what I size to and they just chamber. Accuracy improved bunches after I got that sorted out.

[Bad Ass Wallace]

You can slug the throat though and that is the important bit. Mine are all about 0.315".

Hmmmmm! almost worn out! :kidding:

According to the Small Arms Identification Series by Ian Skennerton "Magazine Lee-Enfield Rifle" Page 29

1) Lee Metford and Lee-Enfield Mk1:
Barrel Specification:
Barrel Length - 30.5"
Rifling Metford - 7 groove L.H. 1 turn on 10"
Rifling Enfield - 5 groove L.H. 1 turn in 10"
Bore Dia - 0.303"
Metford Rifling Depth - 0.004"
Enfield Rifling Depth - 0.0055
Metford Land Width - 0.023"
Enfield Land Width - 0.0936

From book "The British Service Lee" by Ian Skennerton Page 86
2) Short Magazine Lee Enfield Mk 3:
Barrel Specification:
Barrel Length 25.2"
Rifling Enfield - 5 groove L.H. 1 turn in 10"
Bore Dia - 0.303"
Enfield Rifling Depth at muzzle - 0.0065
Enfield Rifling Depth at breech and to within 14" of the muzzle - .005
Enfield Land Width - 0.0936

From book "The British Service Lee" by Ian Skennerton Page 158
3) Short Magazine Lee Enfield Mk 4 & 5:
Barrel Specification:
Barrel Length 25.2"
Rifling Enfield - 5 groove L.H. 1 turn in 10"
Bore Dia - 0.303"
Enfield Rifling Depth at breech - .005
Enfield Land Width - 0.0936


BSA made factory bore gauges in sizes .3025, .303, .3035, .304, .3045, .305. .305 being considered worn out!

It would seem from this data that an Enfield 303 could be .303" over lands and a bore of up to 0.314" at the breech and to 0.316 at the muzzle.

[longbow]

Not sure how one would intentionally or even accidentally achieve rifling with increasing depth of groove from 14" from the muzzle to the muzzle, but interesting nonetheless.

I guess my point was that the largest boolit that enters the throat that you can chamber is most likely going to provide the best fit. If groove depth increase towards the muzzle... well, chances are you will get leading. In that case, a filler may help.

Longbow

[Littlewolf]

i load and shoot a lot of 303brit loaded with the 311414 150gr spire point 34gr 4895 boolits sized to .311

[Mr_yeti]

Best luck I have had is with 170gr RN GC. 11.5gr unique, cci 200. I drop from my mold at .313 and shoot as cast. My enfields are what you might call anomalies however. I have a winchester P14 that slugs out to .312 and a unissued long branch at .3115 both are mirror bore rifles. Groups are less than 2" in cold weather. Havent shot in summer yet.

[Engineer1911]

I have a Long Branch 2-groove that shoots everything cast key holed. Powder or powder charge makes no difference -- every bullet is a profile shoot. Group size is measured in feet. When I shoot jacketed (178 gr) the groups are 1.5" @ 100 yards.

We will now resume normal reloading.

[leebuilder]

Not sure how one would intentionally or even accidentally achieve rifling with increasing depth of groove from 14" from the muzzle to the muzzle, but interesting nonetheless.

I guess my point was that the largest boolit that enters the throat that you can chamber is most likely going to provide the best fit. If groove depth increase towards the muzzle... well, chances are you will get leading. In that case, a filler may help.

Longbow

They back lapped no1 barrels for mk8 ammo. Between the rear sight and the barrel Knox there will be an HV stamp, this is a back lapped barrel.

[leebuilder]

I have a Long Branch 2-groove that shoots everything cast key holed. Powder or powder charge makes no difference -- every bullet is a profile shoot. Group size is measured in feet. When I shoot jacketed (178 gr) the groups are 1.5" @ 100 yards.

We will now resume normal reloading.

Lol. I had fliers and misses too. But I kept at it. Big boolits are the key. What works in my 2 groove savage won't work in my 2 groove long branch. The Long branch is near mint and the savage has a baked chamber.

[leebuilder]

303 is a pillow as full of rusty door knobs to reload j-thinys or cast. Don't let a box of fliers or keyholes discourage you from loading more.
Read as much as you can here, there are a lot of clutched in guys here, and years of experience to draw from
Never think it's a failure, you just know what not to repeat.
We need more info to help you, grooves? Bore size? What kinds of powders?
What range will you be shooting? I have been shooting alot of reduced loads it is easy, fun and very rewarding!!!! When I get a nice group at a hundred meters or more you can't wipe the grin of my face with an axe handle.
Some boolits work better than others.
Have not really found a bad reduced load. For long distance H4198 has served me well
My 160gr Boolits preformed OK, once I started to PC then they really preformed.
Trial and terror.
Need help please ask.
Be safe.

[Le Loup Solitaire]

In the classic text titled "The Art of Bullet Casting" which is still in print by Wolfe Publishing, there is a lengthy article by Al Miller on the 303 British...it includes cast bullet choices, and test firings with various loads. Info on sizing and powders selection as well. Lots of other good info on other calibers and casting. Definitely a good investment. LLS

[MT Chambers]

If your bullet is a "bore rider" you will need it to fit both the lands and the grooves, or you'll get poor accuracy, instability, maybe even keyholing.

[RPRNY]

18-20grns of H4227 is a good start. Pat

Very good advice. 160 grs is rather light/short for the 303. 180 - 210 grs (20 - 22 grs 4227) is better.

[mac60]

They back lapped no1 barrels for mk8 ammo. Between the rear sight and the barrel Knox there will be an HV stamp, this is a back lapped barrel.

This looks like a W.V to me. Could this be the HV stamp you're referring to?

164656

The muzzle of this rifle will absolutely swallow a .314" boolit.

[leebuilder]

Honestly it looks different to me usually across the rifle left to right and farther up and no period it is acompanyed with a SC stamp that means short cone for the pointed bullet, smaller different font. The EY stamp looks like emergency use only stamping.
I will post a pic tomorrow.
Swallows a .314 boolit another member coined that the best "trumpet muzzle"
Be safe

[krallstar]

Check a fired case. Use your inside caliper and get a rough idea of how big a bullet you can fit. My #4 mkII cases measure around .318. I size to .316 w/check. Use the biggest bullet that you can.

[leebuilder]

164781
Not the best pic, the rear sight is removed and would sit just forward the markings. The rust has the foot print of the sight
Be well

[303Guy]

I have one trumpet muzzle barrel. It measures something like .308 (bore) at it's narrowest point about mid barrel. If it won't shoot I'll chop it down to a carbine like my pig gun. Or I'll attempt to lap it parallel. The oversize bore is actually good because there is less case sizing required (none actually) but custom boolits are required (which is fine for paper patching).

I must look for the HV stamp. I'm sure I've seen it on more than one of my guns.

On the topic of fillers, an appropriate filler might solve the trumpet barrel problem by forming a wad behind the boolit. Do the research on fillers before going that route. I favor wheat germ. It's oily and flows well.

[725]

miker19,

Welcome aboard. This site is where you want to be. The shooters, casters, & loaders here are the best around and you will get good advice and eager assistance in your search for loads, molds and such. I caution you to always refer to a reputable load manual like the Lee, & Lyman as has already been recommended. Check and double check recommended loads before you jump and have a good experience with our hobby. Good luck.

[flint45]

Slug your barrel. I shoot two over- eample .312 bore .314 boolit. 18.5 2400 good easy load 185 gr. LEE boolit.

[leebuilder]

They back lapped no1 barrels for mk8 ammo. Between the rear sight and the barrel Knox there will be an HV stamp, this is a back lapped barrel.

Correction : I passed.false info
HV stamp is used on the new rifles (1910) fitted with new mags and new sights, sighted in for the then new mk7 round I said mk8.
SC stamp was for barrels with the new short cone, to improve accuracy with the new pointed mk7 round
Note at the same time circa.1910, 1911 they produced the second variation No1mk3 barrel with the back lap to relieve over pressure from the new mk7 round.
Not all HV stamped barrels have the SC stamp, mostly on post WW2 barrels and comercial barrels.

Toot, toot
Be safe

[Mr_yeti]

Keep at it with cast size. I had a enfield I used to size down .324 mauser cast bullets to .320. Worked in that particular rifle.

[Blue2]

Away from cast for a moment but I loaded up some Hornady .310 diameter 123gr. Meant for the 7.62x39 one time to use them up for a close range tactical 3 gun shoot and they worked very well. Against any sensible expectations on my part out to 100 yards giving 1.5 inch grouping.goes to show that you can be surprised at times but results that defy wha you know should happen.

[longbow]

I am a little late in seeing this resurrected thread...

Engineer1911 if you are getting keyholes I suspect you need a larger boolit and/or filler.

I have been using COW filler for several years with nothing but good results. I started on the advice of David Southall who wrote an article on .303British.com many years ago. He found similar issues with I believe a 2 groove Enfiled and found that filler solved the problem. We e-mailed back and forth a few times so I have kept his e-mails and recommendations and found they worked for me.

I have not had the keyholing issue but did have other things plaguing accuracy so tried COW filler and found it worked well for me. Having said that you can't just drop filler into the cartridge, you have to work up loads using the filler.

I am loading down to 130 gr. boolits in my .303's with good results so keep at it and I am sure you will find a combination that works. Also, I use light to moderate loads with my cast boolits for the most part. I found that when I pushed velocity I was getting skidding of the ACWW boolits in the bore along with gas cutting so had to oven heat treat and quench the boolits to make them hard enough to resist skidding. That worked.

I am usually loading an NOE 216299 sized to 0.315" over 22 grs. of IMR4227 with good results. This is a quite moderate load. if you are trying to reproduce "J" bullet loads and velocities with cast it is much harder to get good results.

Keep trying and good luck.

Longbow

[blixen]

Welcome original poster! You picked a challenging cartridge to begin casting. I've had five so-called .303s that shot jacketed phenomenally well and cast awful. I hogged out the Lee "Enfield" mold to about .315-.316 and got deer-hunting accuracy. I've always wanted to get a new .303 chambered .311 barrel on a good receiver to see how well it would shoot.

[Tedly]

I had a Dutch M95 Mannlicher rebarreled to 30/40 Krag....I opened it up to .303 and it was a shooter. Had a 2 groove 03A3 bbl.....Shot Lee C312-185-1R @ .311 dia w/ 20.5 gr AA5744 under 2" at 100yds . with iron sights.

[Firearm Nut]

I am new to reloading. I have some cast .303 bullets I want to reload but I'm not sure how much powder, what type powder, do I use fillers ect. Any information will help me. Thank you! The bullets are 160 grain and they have gas checks crimped on.

It is sometime useful to know the model of the .303 British you have and if it may have been rebarreled at some point. The later models tend to have truer barrel specs. In the older models the barrel specs can be all over the map. Probably has something to do with a war going on, and they were manufactured all over the map. The P-1914 were manufacture by US firms, and their groove diameters run beyond .311, somewhere between .312 and .314. The older Mk III can vary a fair amount as well. The #4 - Mk I were manufactured later and probably run closest to the .311 diameter, but there is no guarantees. I have and Eddystone P 1914 that I run .314 size 200 gr GC bullets through and it shoots great. I picked up a Winchester P 1914 and it would send the same bullet all over the place, I eventually sized down a .321 PB to .314 and finally got some consistency. To start you certainly set yourself up with a challenge.

[Alferd Packer]

Think about paper patching
Try that before you cut off the barrel.
Paper patching is easy, cheap and it works.
If you never tried it before, please give it a chance.
You can try any type of paper.
I have used printer paper and even the envelopes from junk mail cut up to size.

[Flygrimm]

Think about paper patching
Try that before you cut off the barrel.
Paper patching is easy, cheap and it works.
If you never tried it before, please give it a chance.
You can try any type of paper.
I have used printer paper and even the envelopes from junk mail cut up to size.

AP

I have been considering PPing for the 303. Since I can't get any 4064 for the Garand project for now I might play with the 303 some.

I have a Lee C309-180-R double cavity mold. I'm planning on using some .003" tracing paper to wrap with. It'll be about .321" then sized to .314". I also have a bunch of once fired HXP and PPU brass.

First shots will be out of my N0.4 with a tight barrel or my P14 with a new Criterion barrel.

My questions for you are for paper patching should I use WW or just plain lead with a tiny bit of tin? And what powder could I use? I have plenty of H4895, and N320 with about a pound of IMR 4350 and AA N0. 2. I have a smattering of other powders but there's not really enough of each to play with. Maybe there are some other suitable powders that are actually available that you could recommend.

Stuart

[Jlucas45]

I am new to reloading. I have some cast .303 bullets I want to reload but I'm not sure how much powder, what type powder, do I use fillers ect. Any information will help me. Thank you! The bullets are 160 grain and they have gas checks crimped on.

Welcome to the interesting of cast bullets in th 303...I have been working for a couple of years on a couple of Enfield rifles. I had almost given up on them several times, but a day at the range is better than mowing the grass, so...
Anyway, I have come up with a couple of things that might be of interest. One of my 303's is a "sporterized" Lithgow :( ,but it has a larger bore and I found that the Lyman 314299, and the Lee 160 molds will throw a .314" bullet. Along the way, and a lot! of testing, I found out that BLC-2 is an amazing 303 powder, with a discernible lower resonance. When I started, I was having difficulty keeping 5 rounds on an 8.5x11 target at 100 yards, and now, with much testing, I am getting around 2" on a good day. Other powders that I got fair results were IMR 4895, ACC 2520. I'm not trying to max velocity, and I have always gotten best results at around 1800fps. These are, by observing cases and primers, low pressure loads at around 32.5 gr. and are safe in MY rifes, but use at your own risk.

Have fun!! I did.

[Vonllamawitz]

Cast some .312 using a Lee Mould 155gr with a spire point. I sent Lee a request for 303 load data and hoping to have this to share with the group. Most of the 303 load data is for 180gr and above for boolit weight. I have been considering Longbow's recommendation about 30-40 Krag data and I think it's very sound. I have some Krag data for 160 gr from Lyman that could correspond. I've been a 4064 and 3031 guy with this rifle. Haven't used 4227 or 4198 yet.

[john.k]

Any cast with sharp points need very hard alloy or they will collapse on firing....and collapse unevenly....Ive found if bullets are undersize ,a bit of filler on top of the powder will seal gas leakage ....I use wheat bran as its very light..........undersize bullets will be gas cut and lead the barrel ,and a filler prevents this.

[Vonllamawitz]

John, how fast are you shooting and how did you find out your bullets were collapsing?

[Vonllamawitz]

Longbow, have you used gas checks in 303? If so, how were the results regarding skidding and cutting?

[longbow]

I am lazy and cheap so normally shoot PB Boolits in all guns unless I have to use gas checks or paper patch. I found that in large bore .44 magnum and .45-70 I didn't need gas checks even with hefty loads but with my .308 I did. I tried the Lyman 31141 170 gr. GC boolit barefoot and gas cutting was impressive! I shot boolits in heavy snow so recovered some. Accuracy was non-existant! Gas checks solved that and I got very good accuracy using IMR4227 and cast boolit load data. I didn't push those terribly hard. Rifling twist is 1:12"

When I got the .303's I made a couple of smooth moulds for 200 and 215 gr. boolits which were shot PB over mild powder charges and that worked fine but if I pushed the velocity I got bad gas cutting. So, I bought an NOE 316299 GC mould and made a gas check maker. That helped but if I pushed loads to upper end cast boolit loads I found that I got "skidding" where the grooves in the boolits were wider than the rifling lands in the gun so wide grooves and narrow lands on the boolits and gas cutting. I decided a harder boolit was the next step so oven heat treated and that solved the problem for me.

The rifling twist is 1:10" in the .303's so I figured the faster twist and heavier boolit than .308 was resulting in rotational interia causing the skidding with softer lead. The gas check did not stop that but the gas check and harder boolit did. WQith low to mid range cast boolit loads there is no problem, only when pushing velocity a bit. For me anyway.

Longbow

[TD1886]

I am lazy and cheap so normally shoot PB Boolits in all guns unless I have to use gas checks or paper patch. I found that in large bore .44 magnum and .45-70 I didn't need gas checks even with hefty loads but with my .308 I did. I tried the Lyman 31141 170 gr. GC boolit barefoot and gas cutting was impressive! I shot boolits in heavy snow so recovered some. Accuracy was non-existant! Gas checks solved that and I got very good accuracy using IMR4227 and cast boolit load data. I didn't push those terribly hard. Rifling twist is 1:12"

When I got the .303's I made a couple of smooth moulds for 200 and 215 gr. boolits which were shot PB over mild powder charges and that worked fine but if I pushed the velocity I got bad gas cutting. So, I bought an NOE 316299 GC mould and made a gas check maker. That helped but if I pushed loads to upper end cast boolit loads I found that I got "skidding" where the grooves in the boolits were wider than the rifling lands in the gun so wide grooves and narrow lands on the boolits and gas cutting. I decided a harder boolit was the next step so oven heat treated and that solved the problem for me.

The rifling twist is 1:10" in the .303's so I figured the faster twist and heavier boolit than .308 was resulting in rotational interia causing the skidding with softer lead. The gas check did not stop that but the gas check and harder boolit did. WQith low to mid range cast boolit loads there is no problem, only when pushing velocity a bit. For me anyway.

Longbow

I had a Lithglow Mark III*. I used the Lyman 314299 in it without any problems at all and very good accuracy.

If I were to use a filler I would use the shotshell fillers from BPI and like Longbow said work your loads up using the filler. One of the many things filler does is fools the case into thinking it has a smaller capacity.......see what I mean?

[Vonllamawitz]

Great info! I've just recently started to cast my own. I grew up watching Dad do it and finally started myself. He actually sent me all of his old moulds so they are seeing hot lead again! I know that 303 is a bear to reload for because of all the bore and chamber variations but I've stuck with my Long Branch 44 for 40 years and have figured out a lot about her. Casting is new field for me but I'm excited as finding FMJs can be a challenge sometimes.

[longbow]

I use COW with powdered graphite added for filler in the .303's under PB boolits and find it works well for me though granular fillers is a controversial topic for many. I read about it on .303british.com in an article by David Southall and contacted him for more info. I use COW and like it. Ad TD1886 says, it fools the cartridge (or is it the powder?) into thinking it is smaller capacity. The benefits I see are:

- filler helps protect the base of the boolit (not to the point of replacing gas checks but yes it helps)
- filler "fills" the empty space so loading density is always 100%... there is no chance of a double charge of small quantities of fast powder and it allows use of smaller than normal charges of slow powder because there is no empty space!
- it is quick and easy to "install" using a dipper

Also as TD1886 says, shotgun buffer is a good filler though I have not used it yet, I do have an article from surplusrifle.com on using shotgun buffer under cast boolits.

My opinion is that granular fillers properly used have their place. If you don't like them then don't use them. If you don't want to use them properly then don't use them! You can't just dump filler into an already established load and not expect things to change! I like them and use them.

Yes, fillers raise pressure but as long as it is in an acceptable range who cares? Smokeless powders need a certain amount of pressure to burn properly. Work up your load with filler just like working up a regular load. When you see excess pressure signs like flattened primers stop there and maybe drop powder charge a bit and top up filler.

Back to plain old cast boolits... I find that I get best accuracy when the boolits are just able to fit the throat and all of my .303's seem to like boolits of 0.315" or larger. I have 4 that all seem to have the same chambers and bores and brass interchanges. I have one tight one but it also likes the 0.315" boolits.

I have mostly loaded using Unique for mild loads. IMR4227 for moderate loads and IMR4064 with COW filler for somewhat heftier loads but haven't really pushed the velocity envelope hard. I have also had decent success using paper patched boolits.

Longbow

[Alferd Packer]

Google Lyman 3rd ed.cast bullet manual.
FREE TO DOWNLOAD, GOOD 303 LOADS

[Vonllamawitz]

Any tips on measuring a slug for groove diameter. My rifle has 5 lands which makes it challenging.

[TD1886]

Any tips on measuring a slug for groove diameter. My rifle has 5 lands which makes it challenging.

Make yourself a little V tool out of a strip of metal at least 1/8 inch thick and about 3/8 inch wide and say 1 inch long and bent it into a pefect 90 degree angle. Then file that outside sharp tip flat so that the V will sit perfect upright on a table. Don't file so far through that it weakens it. Now take your regular 0 to 1 inch mic and measure something of a known value such as a new 308 bullet. Mic the bullet first to see exactly it's measurement. Then put that V block on one of your mic's anvils and measure that 3/8 bullet again. You'll get a larger reading. Subtract the 308 bullet's measurement from that larger reading and that number will be your constant. Now measure a lead slug you forced through the bore of your 303 barrel. You will see how the V block only contacts the groove bands. So you don't get mixed up on a bullet thos larger outer grooves are grooves in the barrel, and the deep inset grooves on the bullet are your lands in the barrel. Substract that constant number from your reading and that is the diamter of the bullet your forced through the barrel or in other words your groove diameter. Here's a pic of the V block I drew.

314290

[WinchesterM1]

My go too load for the 303 is a NOE 316299 Powdercoated GC sized to .316, over 25 grn of IMR 4198 and it shoots at point of aim, i have no idea what the velocity is, but it hits hard. I will probably hunt with this load this season.

[ukrifleman]

Any tips on measuring a slug for groove diameter. My rifle has 5 lands which makes it challenging.

Wrap a piece of copy paper once around the slug and take a measurement, then measure the thickness of the paper and subtract twice the thickness.
This will give you the groove diameter.
ukrifleman

[groovy mike]

16 grains of Alliant 2400 or 13 grains of Red Dot without any fillers should make mild target loads that would also be suitable for hunting under 100 yards or so.