Bullet fitting question

[Mr_Flintstone]

The other day I was reading about bullet options for 6.5 Carcano rifles. One of the articles mentioned using standard .264" bullets instead of bore filling .267-.268" bullets to reduce the pressure in older rifles. I can see where you would have gas leakage around .264 bullets, especially boat tail bullets that don't obturate, but how much pressure would be lost in this way? Is there a way to calculate the size of the gap through which the gas would leak and/or a way to enter that into Quickload or GRT?

 

[.38 Special]

I'm not sure that gas leakage is behind the pressure reduction, but rather the effort it takes to engrave the bullet. Either way, I wouldn't have the slightest idea how to calculate the pressure reduction without empirical testing.

 

[Varminterror]

This practice is one of those old school "we just tried something that should work in theory, and nothing blew up" type of things. We know it reduces pressure, but everything else is really just "we know the bullets come out of the other end".

Of note, it's also simple enough to just use reduced loads to reduce operating pressures.

 

[WestKentucky]

Less a question of will it work but more a question of how well it works. Dropping .002 in diameter your still catching rifling so your going to have some degree of stability. When you start considering the tolerances of military rifles you begin to wonder though. Some of those bores are grossly oversized and still shoot decent groups.

The question of pressure loss is interesting and would be a function of exactly how undersized the bullet is, compared specifically to the barrel it’s being shot through. I suspect that the gas flowing hot and fast around the bullet would cause rapid erosion of the throat and likely to a lesser extent over the whole barrel.

 

[AJC1]

It seems to work well enough for the Sammi nominal bore size between 43 mag pistol and 43 mag carbine. Success is found with jacketed bullets, and I expect you would run into the same leading and accuracy issues that guys have when trying to shoot cast in the carbine. Lee did make a few molds specifically for that rifle, but accurate molds can do it any time you need.

 

[GeoDudeFlorida]

The other day I was reading about bullet options for 6.5 Carcano rifles. One of the articles mentioned using standard .264" bullets instead of bore filling .267-.268" bullets to reduce the pressure in older rifles. I can see where you would have gas leakage around .264 bullets, especially boat tail bullets that don't obturate, but how much pressure would be lost in this way? Is there a way to calculate the size of the gap through which the gas would leak and/or a way to enter that into Quickload or GRT?

A Carcano should have a .257” bore and .268” groove but, as my Great Uncle used to say, “There was a war on,” so yours might be tight, loose, or both. Maybe even neither.

Gas leaks and pressure changes are going to be impossible to measure without strain gauges and pressure testing equipment. Try a few and see how it goes.

 

[Mr_Flintstone]

A Carcano should have a .257” bore and .268” groove but, as my Great Uncle used to say, “There was a war on,” so yours might be tight, loose, or both. Maybe even neither.

Gas leaks and pressure changes are going to be impossible to measure without strain gauges and pressure testing equipment. Try a few and see how it goes.

I actually meant groove. I was having a little abdominal distress when I wrote the post, which later turned into some kind of Chinese spy balloon stomach virus.

I’m not trying to develop a reduced load by using a smaller diameter bullet, but more so just interested the concept of gas leakage. I started feeling better around lunch today, and dug out the slug where I tested the bore. The bore measured .257” and grooves measured .268”. The average land measurement was .080” and groove measure was .130” @ 4 each. This would be slot of roughly (4 x .130”) x .004” = .002 square inches. I was thinking I could put a rectangular gas leak into GRT to approximate the leak through the grooves and then make up some matching loads and run them across the chronograph to see how they match. That’ll have to wait for another day though since I don’t have any .264” bullets right now.

 

[Bazoo]

It's pretty common to have jacketed bullets be .001 smaller than groove diameter, and accuracy is not degraded. I don't know how much smaller you can go and keep accuracy. That'd be an interesting experiment.

 

[stringnut]

Huge amount of variables in doing a calculation. Pressure curve, peak pressure, shape of the bottom of the bullet, bearing surface, ogive, barrel condition and probably others. However they may be so small as to be insignificant. Not an engineer by any means.

Results of testing could be interesting.

 

[AJC1]

I expect the leakage to be shaped like the pressure curve for ignition not linear at all. Lube and operation helping to a point, and then fall off a cliff...