Free Bored chambers

[Greg5278]

I need some info on the design of Free Bored chamber design. I am designing one for a slug gun. I am thinking of a 45 degree angle at the case mouth before starting the freebore. What diameter should the freebore be, groove diameter, + or - ? I think the freebore should be 1 caliber in length, before starting a 2.5 to 7 degree leade into the rifling.

My current custom chamber is showing pressure long before it should, even with an 8200PSI load in the 12 gauge.
Any help would be appreciated, Thanks Greg

[Scrounger]

I need some info on the design of Free Bored chamber design. I am designing one for a slug gun. I am thinking of a 45 degree angle at the case mouth before starting the freebore. What diameter should the freebore be, groove diameter, + or - ? I think the freebore should be 1 caliber in length, before starting a 2.5 to 7 degree leade into the rifling.

My current custom chamber is showing pressure long before it should, even with an 8200PSI load in the 12 gauge.
Any help would be appreciated, Thanks Greg

In shotguns that area in front of the cartridge is called the forcing cone. You might ask some of the shotgun boards for information on that.

[StarMetal]

I think 45 degrees is too much of an angle.

Joe

[felix]

What you are talking about here is strictly up in the air because of the many different ideas, and anyone will work great under certain circumstances. Circumstances such as: better consistent accuracy now for a low number of shots (the BR game), or a little less consistent accuracy for quite a few shots (the military rifle game), with up to a factor of 4 or so. In one of the magazines, I think Precision Shooting or Handloader, or somewhere else maybe, someone compared the 243 with the 244. The total difference of any account was in the throating design according to the author. That would be a must read for you before a final decision. ... felix

[Greg5278]

I know the angle in a shotgun is called the forcing cone. The issue is the missalignment of the slug in the forcing cone into the rifling. The 45 degree is just ahead of the case mouth for maybe .040". Greg

[felix]

I realize that, Greg, but you are shooting a boolit for real here, not a bunch of shot. ... felix

[wills]

you see 11 degrees cited often for handguns

[Johnch]

I have never done much with a rifled slug barrel.
I am (not much lately) a trap ,skeet sporting clay shooter , due to back problem I am shooting more CB in rifles this past year.

8 degrees is normal for a lengthened forcing cone.
I seem to think 15 -20 degrees is normal for shot barrels ( mind is sliping , don't quote me on this) from the factory.

I got my reamer from Brownells ,they sell several.
I don't know how much they are now , but I paid $150 and sold it for the same 2 years later.

Johnch

[Greg5278]

The forcing cone is generally .798" at the end of the case, and tapers to bore diameter in .451". That is the SAAMI spec chamber. The current factory, and custom slug guns all use this setup, with the exception of the Tarhunt. The reason I ask is that I am trying to wring the accuracy out of the shotgun slug, i have already fired 1" groups for 3 shots at 50yards. The problem is the slug is not engraved concentric with the bore. I am shooting a 775grain trucated cone bullet with a .500" metplat at 1400 fps. I have taken 2 deer with it, but nothing larger. Thanks for the help

[Buckshot]

"............The problem is the slug is not engraved concentric with the bore."

And there is the difference between throat theory for a boolit to be spun, and a load of shot. With a load of shot, they're trying to gently herd all these single minded bits of shot into a cohesive group as they move down the barrel.

I guess you've recovered fired slugs to see that they've engraved a bit off kilter? Are these being fired out of paper hulls, plastic or brass?

A freebored chamber is basicly a chamber with a long throat. Weatherby used them to allow the bullet to get a running start before engraving to help keep pressures down. Instead of pressure building to finally break the case's grip and overcome enertia, then a micro second later it comes up against the lands. Or even has to overcome engraving along with case grip and enertia.

The throat should ideally be as tight a possible to the boolit to provide guidance to it, and most are about .001" over, or .0005" around the slug. That is, those rifles that DO have much of a throat.

"............. I need some info on the design of Free Bored chamber design. I am designing one for a slug gun. I am thinking of a 45 degree angle at the case mouth before starting the freebore."

Some rifles have such a feature rather then a 'simple ledge' at the end of the casemouth. I think it's a good idea myself, vs having a sharp abrupt beginning to the throat.

"............What diameter should the freebore be, groove diameter, + or - ?"

See above.

"I think the freebore should be 1 caliber in length, before starting a 2.5 to 7 degree leade into the rifling."

One caliber is actually quite long unless the slug is well guided while it is in the freebored area.

"............The reason I ask is that I am trying to wring the accuracy out of the shotgun slug, i have already fired 1" groups for 3 shots at 50yards. The problem is the slug is not engraved concentric with the bore. I am shooting a 775grain trucated cone bullet with a .500" metplat at 1400 fps. I have taken 2 deer with it, but nothing larger."

Are we talking about 2 different rifles (or shotguns) here? I see nothing at all bad about 1" three shot groups at 50 yards from a rifled shotgun. If it were me and I got that with slugs engraved off center I'd still be tickled.

"............My current custom chamber is showing pressure long before it should, even with an 8200PSI load in the 12 gauge."

Is that a load out of a book that shows 8.2K PSI with shot, or with a slug? If it's for a load of shot, a solid slug has no wad cup to collapse as it begins to move the shot, the shot also allows a bit of movement while it 'forms up', and the shot doesn't suffer engraving.

Further, I have no idea what your 'Current Custom Chamber' looks like, what OAL it will allow, nor the design of the boolit you're shooting from it.

...............Buckshot

[StarMetal]

You might want to use brass cases, then go to a slightly larger diameter bullet, then it won't be wobbling around so much in the freebore. It will then be more liked fitted to the throat as we do for cast in rifles. Generally rifled slug barrels have shallow grooves too. I did alot of experimenting with an H&R rifled slug gun in 20 ga. I use to have the Lyman wasp waist slug mould for it that used the WWAA wads. I had it scoped and I got that one inch group at 75 to 80 yards. Eventually I designed a copper jacketed slug with no plactic shot cup and I had it shooting really good. At that time I lived in a shotgun only state for deer gun season and have since moved and sold all of it.

Joe

[Willbird]

I agree with Starmetal here (isn't THAT a scary thing) and I will add some of my own ponderings, I think oversize/undersice of bullets to throat would follow many other things and operate based on percentage or a proportion of some kind, IE an .800" bullet that is .001 bigger than the throat is not as much oversize in the real world as one that is .310 dia and .001 bigger than throat.

one handicap you have is that you are actually working with a pistol type bullet, IE it ias one caliber or less of bearing area. It also may bump up quite a bit while it is still in the case, swelling the case right along with it, then be swaged back down upon entering the forcing cone/throat/bbl.

The Lyman 12 gauge air rifle pellet style slug for example when cast soft is totally re-shaped when it is fired...what comes out of the barrel has no resemblance to what was loaded into the ctg. case.

If I was going to fiddle with what you are doing I would make a machined brass ctg. that has .001-.003 to expand to fit the chamber of the gun...I would put an index mark on that case to put it in the chamber the same way each time, and I would use a shotshell reloader right on the shooting bench to reload the ctg. I would use a uniflow or other "real" powder measure for the powder charge....and I would cut my case for Large pistol primers.

Once you got THAT shooting, you could work on isolating other variables and seeing what could be made to work as to regular shotgun hulls, purchased brass cases etc.

But hey I like to play hehe........I have run my mind down the road you are going many times (Ohio is a shotgun only state for long gun deer season). FWIW I think a well built 410 would really work the best because of being able to use longer bullets without needing a shoulder transplant.

Bill

[Greg5278]

Buckshot, the 1" group was fired with another shotgun with a custom rifled barrel on tghe same type of action. I have recovered hundreds of slugs that bear out the lack of concentrisity in the bore. Most slugs are too mashed to discern the problem, but mine it far more durable than factory ammo.

I made a parrafin cast of the chamber to see exactly what the chamber looks like. I know it won't do for a measurement, but it helps. The chamber ends abruptly afgter the case mouth. Then the gentle leade starts. The person chambering it forgot to freebore it. There is no expansion room ahead of the case, before the rifling engagment.

The load is for a non-canister powder, and has been pressure tested in a laboratory pressure cannon. It reads between 8000-8200PSI. The chamber is drastically increasing pressure, long before it should. I also have a hotter load with the same powder, that develops 11,200PSI, and has no pressure signs in any guns.

The chambering was a custom job, and used two operations. It was done by a relative. He simply forgot to run the freeboring reamer in, after the rest of the chamber was cut.

I am trying to use the fundamentals of a good cast bullet chamber for this project. My shotgun, is more rifle than anything else. It is fully threaded, and free floated to the action. The trigger is lightened. I am basically shooting a Bore rifle, like the fully rifled 12 bores made by Westly Richards.

This gun will never be used to shoot shot, only my handloaded slugs. I have designated slug guns.
Greg

[9.3X62AL]

Greg--

My slug work to date has been in smoothbore barrels and service-grade 870's with Fosters, so a lot won't apply to your endeavors.

Soon after a bank robbery debacle in May 1980 that our agency (and subsequently several others) engaged in, the decision was made to upgrade our patrol shotguns a bit from their 18"/bead sighted barrels, and to issue slug rounds along with the Rem 870 rifle-sighted barrels. I was asked to be one of six shooters to do a three-day shooting test with these critters, and probably fired 400 slugs personally during that testing bit. I learned a lot--especially to not volunteer for slug shooting sequences again. Kidding aside, the Rem 870's would pretty reliably hold 2.5"-3" @ 50 yards, and would do 6"-8" at 100 yards. One condition that would REALLY screw up patterning was to tighten down the barrel retaining cap completely. Slugs went galley-west in every shotgun so conditioned, and after the tension was re-set to the factory-recommended "one detent click backed off from snug", accuracy returned. Don't ask me.

Our conclusions were that for human-form targets, the slugs were at their best within 75 yards, given the radial dispersion observed throughout the test sequence. We recommended 223 caliber self-loading rifles to supplement this essentially short-range long arm--and the idea slowly took hold over the years, prompted at last by the use of bead-sighted 14" barrels on our 870's that were needed to fit around the airbags in later model Crown Vics.

So, if I were to get 1" groups at 50 yards from a slug gun--I'd be ecstatic. To 75 yards or so, I would be very confident with my rifle-sighted 870 and Foster slugs on deer. Given the soft lead used in most slugs, I'm not sure how you will avoid the distortion you mention to the slug sidewall other than to employ bench-rest-like ammo engineering that you seem to be undertaking. While I enjoy "The Chase" as much as anyone, on a practical level for deer hunting purposes, I would say you have exceeded the need already. Good work, BTW--reloading slugs successfully is a problematic enterprise, and I think you've done a LOT better than I could ever do.

[Greg5278]

Most of mt slugs are solid base, and need rifling for stabilization. I have yet to find a hollow base slug that flies straight without yaw in a rifled barrel. I have fired groups that are better than neccesary for hunting. My belief is that the gun, and load should be the least of your worries in the shooting equation. The conditions, lack of a rest, and huge recoil energy are not always conducive to good accuracy. I fired one shot at a buck offhand at 85 yards, and he dropped in his tracks. The gun, and ammo were not a concern. I fired a group in low light conditions and open sights with my Ztruncated cone bullet last year that was about .750" for 3 shots.

In the 12 gauge sabot guns, I have fired some .750-.875" groups at 100 yards. I use sabots, when the shots are long, as in over 100 yards. I prefer the full bore cast slug. As for reloading slugs, it is very frustrating, and I have fired thousands of rounds to get to this point. I created most of my loads frokm scratch, consulting loading manuals, and using the pressure cannon as a resource. Greg

[Char-Gar]

I am working on some very old memory here, but IIRC a true freebore is a rare bird. Some of the hypervelocity work done in Germany by Gerlich and perhaps others used a true freebore. A true freebore allows the projectile to leave the case before it impact the rifling.

This is somewhat difference from the long throat that Weatherby and others used. The bullet did not entirely leave the case before it hit the rifling. The purpose was the same and that was to lower the pressure. It was sorta of a silly thing. I worked with a 300 Weatherby back around 1960 and it lacked the Weatherby long throat. The Weatherby chamber did reduce pressure, but it also reduced velocity and accuracy. A standard chamber for a Weatherby round could be loaded down a few grains from what Weatherby used and the pressure and velocity would be the same as the Weatherby and the accuracy was better.

I would think a true freebore should be as close to barrel groove diameter as possible to keep that bullet straight so it impact the rifling dead on.

To each his own, but freebore never had any popularity in the run of the mill rifle and I see no practical benefit..only potential problems from it's use.

[Logan]

My CZ527 (.223) is a genuine freebore then.

I would have to seat a 63 gr ex military projectile well out of the case to engage the rifling.

The freebore just seems to merge with the bore diameter.

Shoots sub inch.

[Bass Ackward]

I am working on some very old memory here, but IIRC a true freebore is a rare bird. Some of the hypervelocity work done in Germany by Gerlich and perhaps others used a true freebore. A true freebore allows the projectile to leave the case before it impact the rifling.

This is somewhat difference from the long throat that Weatherby and others used. The bullet did not entirely leave the case before it hit the rifling. The purpose was the same and that was to lower the pressure. It was sorta of a silly thing. I worked with a 300 Weatherby back around 1960 and it lacked the Weatherby long throat. The Weatherby chamber did reduce pressure, but it also reduced velocity and accuracy. A standard chamber for a Weatherby round could be loaded down a few grains from what Weatherby used and the pressure and velocity would be the same as the Weatherby and the accuracy was better.

I would think a true freebore should be as close to barrel groove diameter as possible to keep that bullet straight so it impact the rifling dead on.

To each his own, but freebore never had any popularity in the run of the mill rifle and I see no practical benefit..only potential problems from it's use.


Chargar,

What you are describing is a freebored throat is not all that uncommon. It fits the description of a line bored wheeler without a BC gap.

Or a Taylor throat in a wheeler where a freebore is cut in the barrel of a caliber to a caliber and 1/2 so the bullet exits everything and then aligns before it hits the rifling.

So I don't think the question should be if a freebore will work, but would it be preferable to an ordinary throat. Maybe in an action that is not capable of handling high pressure, it would give a velocity advantage. I have a preference for the match type chamber in 22s where the bullet fully engraves when chambered. But this is not possible with most shotgun actions.

[Char-Gar]

Bass..You are of course correct. My mind was in rifle mode. The jury is out on the Taylor throat. I know Jim Taylor pretty well and it was his dad who came up with the idea. The notion being the bullet will enter the rifling dead on after that trip down the tunnel in the barrel. Sixguns seldome align perfectly so the Taylor throat is supposed to cure this and help accuracy.

The line bored cylinder is supposed to correct this bullet hitting the rifling at an angle business as well.

I know folks with line bored sixguns and sixguns with Taylor throats. Some swear accuracy is better and others say it is the same. Tests with the Taylor throat show a slight increase in velocity.

Sixguns is sixguns and rifles is rifles and while they share some common traits, the rules don't aways cross over. sometimes they do..sometimes they don't.