Reboring barrel: minimum difference

keithhagan · Jan 16, 2021

For anyone with knowledge or first hand experience, when reboring a barrel to a larger caliber, what is the minimum increase in caliber that reputable shops will require?

Under ideal circumstances with perfect tooling and machining, so long as the bore diameter of the new, larger caliber is greater than or equal to the groove diameter of the current, smaller caliber, it should be possible to rebore. However, I imagine that some allowance for tolerances must be made.

For example, per SAAMI the groove diameter for 9mm/.357/.38 caliber barrels is .355 and the bore diameter for 9.3mm caliber barrels is .358. Is a 3 thou difference enough of an allowance? If not, what do shops actually require in real life?

steelslaver · Jan 17, 2021

I would simply fire the 358 bullets down the .355 bore and never worry about it even a little tiny bit. Guys regularly fire 454 or larger bullets in large throat 45 s down 452 bores and that is a lot more bullet mass to squeeze down.

Gun experts like PO Ackley did testing where they fired bullets as large as 357 down .308 bores with guns that had been chambered for the larger bullet with no high pressures. The bullet is instantly swedged to bore diameter and away it goes.

The actual problem with something like say a 30-06 when loaded with say 312 bullets isn't the 30-06 308 barrel, but its chamber. The neck of the chamber is set for a 308 bullet in the brass's neck, when a .312 bullet is laded and chambered by the camming action of a bolt action the brass neck is very tightly squeezed onto the oversized bullet and it doesn't release when fired causing a huge initial pressure spike. IF the neck portion of the chamber was opened up by an additional .004 this would not occur.

This could cause a problem in a semi auto handgun, but most likely the action would not chamber the round if the neck area was swelled that much by oversized bullet. But, once again that is a chamber problem not a bore problem. The chamber could be adjusted for this and the 355 bore would work. You might in counter a problem if your loads were right on the very edge with the correct bore diameter, but then your skating on very thin ice with the correct bore anyway.

In order for a shop to make a .355 barrel into a 358 barrel they would need to match the twist rate and land, groove dimensions perfectly plus start the rifling button or cutter almost perfectly in the existing lands and grooves. Not going to happen.

What is your idea anyway???

WR Moore · Jan 17, 2021

keithhagan said:
For example, per SAAMI the groove diameter for 9mm/.357/.38 caliber barrels is .355 ................

I don't presently have access to SAAMI documents. However, I have to question if that's a minimum or nominal dimension. What's normally seen in barrel production is a range of about 0.001 in on either side of a nominal diameter. So your .38/.357 barrel is a nominal .357 in groove diameter with a range of .356-.358 in. 9 mm is generally considered to be .355 in nominal, range of .354-.356 in. So, you can't count on any particular barrel being XYZ groove diameter.

I should also note that, at least 70 or so years ago, Colt was famous for making .38 Spl/.357 barrels with a groove diameter of .354 in. in at least some models (Official Police and Python IIRC). I can't find my reference book and can't recall if that .354 was minimum or nominal dimension.

You're also making leaving out some other considerations in your process. First is straightness of the bore, probably not an issue in a handgun barrel. The other is that you assume that after the boring, one goes immediately to rifling. One does not. There is generally a honing step to produce a uniform, fine finish and dimension to what will be the bore of the new caliber if one is cut rifling. If one doesn't get an acceptable dimension and finish after honing, one goes to the next acceptable caliber. There's also some other practical machining considerations.

Steelslayer makes good comments on the practical nature of what you want to try to do.

austintexas · Jan 17, 2021

I once had a Highway Patrolman 357 mag barrel converted to shoot .45 Colt with no problems. Revolver recoiled pretty good with that light barrel.
SWCA 892

SMSgt · Jan 17, 2021

What are you trying to convert from/to? Might be easier and cheaper to buy a new barrel.

keithhagan · Jan 17, 2021

steelslaver said:
I would simply fire the 358 bullets down the .355 bore and never worry about it even a little tiny bit....

Oof, I think you misread my original post. Unfortunately, that misreading tricked you into assuming that my question wasn't a thoughtful one and led you on a detour in your response.

My question was, in general, what is the minimum difference in caliber for reboring and rerifling? The reference to 9mm/357/38 and 9.3mm calibers was made to serve as an example using two calibers that are very close, but whose groove and bore, respectively, do not overlap.
The 9.3mm caliber is not .358" caliber. The 9.3mm bore diameter is .358", not the groove diameter. The groove diameter is 9.3mm that is why it uses a 9.3mm (~.365"-.366") bullet, not a .358" bullet.
On the other hand, had I been silly enough to ask specifically about reboring/rerifling a 9mm/.357/.38 barrel to .358 caliber, I would agree with the incredulity in your response whole-heartedly.

keithhagan · Jan 17, 2021

WR Moore said:
I don't presently have access to SAAMI documents. However, I have to question if that's a minimum or nominal dimension...

The (nominal) groove dimension for each of 9mm/.357/.38 is .355" per SAAMI. I agree with your point about what groove diameters are in real life, see the last sentence of the second paragraph of my original post. Likewise, your other points should be addressed by that same sentence.

Unfortunately, I do not agree with your estimation of the value of steelsaver's comments.

steelslaver · Jan 18, 2021

Well you could ream a barrel to the groove diameter and rifle it. With cut rifling you would end up with the lands at reamed diameter and groove at depth cut. But, if you used a button type rifling you could make the land diameter smaller than what the diameter it was reamed to as this method does not remove metal it displaces it. Say you reamed a barrel to 361. Then you pressed a button down the barrel it could press grooves into .365 and the metal displaced to form the grooves would form the lands and make the land to land diameter say .357. No metal is removed just moved. Cut rifling of course the land to land is what ever you start at before you lap the barrel which will only remove a very small amount.

My dad made his own muzzle loaded barrels. I have a couple button setup just never got around to trying them to many other gun projects.

If I wanted a 9.3 barrel myself I would simply start out with some 9.3 barrel and make it what I wanted it to be. . But, I personally believe you could shoot a 9.3 bullet down a 357 bore anyway if your chamber was right and you made longer than normal forcing cone of in an auto a tapered throat to swege the bullet down. It just' take that much force. I wanted a .312 barrel for a 327 Mag I took some .312 barrel and made itt fit a K frame smith and then just used an old barrel as a shroud.
K frame 327 project

2152hq · Jan 18, 2021

keithhagan said:
For example, per SAAMI the groove diameter for 9mm/.357/.38 caliber barrels is .355 and the bore diameter for 9.3mm caliber barrels is .358. Is a 3 thou difference enough of an allowance? If not, what do shops actually require in real life?

Any of the rebore shops I've delt with over the years would not do the above.
Some might, but they'd want to carefully measure the actual barrel bore before committing to the job.
SAAMI specs are one thing, real world is that specs vary... and vary a lot especially in the 2 calibers listed.

Taking a true .355 groove dia bore out to .358 and then rifling it from there to .366 for a 9.3caliber cartridge can work. But in my experience working with some rebore shops and on other's projects most rebore shops would choose not to do the job if offered to them.
SAAMI specs are one thing but real world specs are another thing.
That .355 may actually be larger and fail the attempt to get a clean rebore to the needed .358 for the new bore dia of the 9.3cal.

If with careful measurement the 9mm groove dia (.355) was in fact small enough for the entire length to allow clean up to the needed .358d for the groove dia for the 9.3cal,,then it may be something one of them may do.

Most rebore shops will want to go up at least 'one full caliber' as a very simple and sloppy way of putting it.
It's not a very well worded rule.
Experience of the person doing the work knows right off wether they should do it or not.
A 9mm usually will go minimum up to a 9.5mm (.358 to a .375)

Talk to the person running the machines,,doing the work.
Also needed is the dia of the barrel muzzle.
Most shops have a dia minimum they will work as respect to caliber. Both dia and actual cartridge can be in play in this last spec.

WR Moore · Jan 18, 2021

keithhagan said:
The (nominal) groove dimension for each of 9mm/.357/.38 is .355" per SAAMI. I agree with your point about what groove diameters are in real life, see the last sentence of the second paragraph of my original post. Likewise, your other points should be addressed by that same sentence.

Production tolerances must be taken into account. Also, the barrel material can make a significant difference as stainless does not machine like carbon steel. This can make a significant tooling difference in precision work.

Posters above have make a great suggestion: contact firms who do this work and ask them, but realize you're going to have to be specific about materials.

Reboring barrel: minimum difference

keithhagan

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steelslaver

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WR Moore

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SMSgt

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keithhagan

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keithhagan

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2152hq

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