Bullet / bore / groove fit

[cuslog]

So I've been primarily a cast bullet / target shooter all my life. In that "game", bullets are typically .001 ~ .002 (or even more) over groove dia. in the barrel. But this is typically a lower pressure, more forgiving "game".
Now, thinking about jacketed bullets and higher pressure cartridges, what would be acceptable "fit tolerances". I'm thinking that a bullet fit that's over groove diameter may not be as forgiving as in the case of a cast lead bullet. Are jacketed bullets "precisely" at groove diameter and no more ?

 

[GGaskill]

Are jacketed bullets "precisely" at groove diameter and no more?

Generally, this is the case. There was some variance in old cartridges but as manufacturing became capable of closer tolerances, they were accepted.

 

[trevj]

It is well worth reading the books written by some of the old school gun cranks and seeing what they actually did as experiments.

I think most of them, would send a Company Lawyer into convulsions, and seeking a "safe space"!

There are written accounts of guys steadily increasing the bullet size WELL beyond the bore size, and yanking the firing cord from behind cover, only to find the rifle action quite OK.

Stuff of the era of Hatcher's Notebooks, and similar.

Wish I could recall who it was that did the number on a tired old High Wall, but they finally managed to blow it up by stuffing as much of the fastest Pistol powder they had, into it, compressed, and reaming the chamber several sizes larger than the bore of the original chambering. They did have to cock the hammer with a screwdriver, as one of the shots broke the thumb tab off!

From what those guys did, you should think, in the modern view, that any deviation from 'exactness', would result in uncontrolled chaos should have seen them dead and gone as a result of their experimentation, but they lived to tell!

Results downrange count, if they are repeatable!

From my readings online, while chasing down Swaging info, you give up on Thousandths of an inch, and start working in tenths of a thou, or less, if you can afford the measuring equipment. Guys report that they can literally tell where in the run a bullet is, by it's size, as the early ones, while the dies are 'cold' are smaller than those that were made once the dies warmed up from use.

 

[GGaskill]

P. O. Ackley was one of those noted intrepid experimenters.

 

[CalG]

Obturation being a real thing....

Free bore comes to play even on jacketed bullets.

Damn erosion!

 

[MilGunsmith]

Bullet core composition and hardness make a difference in the fit also. If you notice, the monolithic bullets like Barnes X, Hornady GMX, etc. all have grooves for the materiel to displace into from engraving into the lands. If the jacketed bullet has a tungsten or other hard core, the same situation happens. We did soft catch of jacketed bullets, and found that with a tight bore an open based FMJ bullet had some lead displacement out the base, whereas an OTM bullet, like a Matchking, had the lead displaced forward within the jacket. It was also found that the odd grooved barrels did less internal disruption than the even grooved ones. The thought being that the non opposing lands did not compress the bullets as much.

 

[codeine01]

Were these observations by your testing/company about odd-numbered land/groove barrels causing less obturation/lead-displacement connected in any way to how the popular 5R barrel rifling came to be? Just curious, I don’t know much about the history of that other than supposedly it’s accuracy gains.

 

[kenton]

I actually have wondered this myself. I have a 91/30 Mosin Nagant made in 1942 with a terrible barrel. I have daydreamed about rebarreling it and making it look as original as possible, the problem is that the only 29"+ barrels I can find are .308 dia barrels. I believe the Finns rebarreled them with .308 dia barrels but recommended only using Russian .310/.311 ammo in emergences. I was also under the impression Ruger used .308 dia barrels on the Mini 30 when it came out.

I would probably mostly shoot reloads with .308 bullets but I don't want to leave a ticking bomb waiting for someone to load it up with spam can ammo.

 

[GGaskill]

I built an SKS with a .308 barrel and marked it .308 x 39. I used a custom chamber reamer with a shallow taper throat to address the potential for .311 bullets although I always intended to use .308 bullets myself.

You could do the same thing, although marking the barrel won't help much unless it is not under the stock/hand guard. I would mark it 7.62 Russian/.308 which should get peoples' attention.

 

[CalG]

I have reloading dies for both .310 and 308 x 39.

Cast projectiles, it's not obvious which is less accurate out of the stock barrel. ;-)

and to add

Long throating is "some folks way" to overcome ruptured primers that lead to stuck firing pins.

 

[john.k]

Ive fired 303 military ammo out of a 95 Winchester in 30/40 ,303 military full jacket bullets out of a M17 in 30 /06,and 303 full jacket bullets out of a Garand M1 ....all these bullets would be .312/.313 dia...and no signs of pressure any different to normal.

 

[MilGunsmith]

Were these observations by your testing/company about odd-numbered land/groove barrels causing less obturation/lead-displacement connected in any way to how the popular 5R barrel rifling came to be? Just curious, I don’t know much about the history of that other than supposedly it’s accuracy gains.

Yes, this testing was part of the M24 Sniper rifle project. We needed to find a barrel that would shoot M80 ball in addition to M118 and M852. The Brits have used a 5-groove barrel for years, going back as far as the .577cal. Enfield musket. They continued using it in the .303 rifles with the original long round nose 215gr bullet.

 

[1yesca]

remember the old rifleman story about the guy that had a jap 38 that was re chambered to 30-06 ? now how and with what i don't know as

it was not done with a 30-06 reamer as the 30 cal. pilot would have never fit the 6.5 bore and if it was ground down how did the bullet

fit and the bolt close ? maybe they used a drill bit and drilled tell the bolt closed on a round but one thing is for sure it was done

and the rifleman did a story on it back in the 60's and that 30 cal bullet look like a hotdog after it came out that 6.5 barrel it was

only by the grace of god that fool keep his life i don't think that stunt could be repeated with out the barrel or action coming apart

 

[GGaskill]

I'm certainly not recommending this stuff but as long as they were not firing AP, .30-06 in the government loadings is not a high pressure round. And according to Ackley, the M38 was the strongest action out there.

 

[john.k]

In WW1 the Austrians converted Mosins to 8x50 by rechambering with a long throat....that is a 325 jacket bullet in a 312 groove bore