[SciFiJim]

So much pressure that it blew out the first posters remarks!

[SquirrelNuts]

I don't know why it did that. I am not sure where my post went.

I had a very long well thought out post, and now it is gone.

The short version is - how much larger must a jacketed bullet be (in thousandths of an inch) before dangerous pressure starts to build up due to the larger size.

My original post was about four paragraphs of good discussion. Sighhhh.

[SquirrelNuts]

I know that a jacketed bullet can be about 0.001" larger than the bore and be ok, and a lead bullet can be about 0.002" - 0.003" larger than the bore and be ok. At what point does it get too large that it causes dangerous pressure that could damage the firearm?

I got to thinking about Finish Mosins and most of them having a 0.308" bore, and the mil-surp 7.62x54R ammo I have measures 0.311" in diameter.

[Jim Watson]

When it gets so big that the case neck is tight in the chamber and the brass cannot expand to release the bullet. Clark calls it "Pinch".

P.O. Ackley once chambered and throated a .30-06 barrel for .35 Whelen and said it did not give any indication of excess pressure. He said the bullet would be sized to fit the bore by the time it traveled its own length which was before maximum pressure. Then there was the famous 6.5 Arisaka rechambered but not rebored to .30-06; sent to and reported on by NRA.

[SquirrelNuts]

What happened to the Arisaka?

[Shimitup]

I think that depends on where your load is when you introduce the larger bullet. Here's my story about loading some defective Sierra .357 150 JHC's back in the early 80's. I had worked up a load that had become my favorite for my Ruger Security Six, I considered it a slightly heavier than moderate load leaving a noticable radius on the edge of the primer cup and around the firing pin, not much flex ahead of the web and easy to eject. I had just conjured up my favorite load with a fresh stock of the Sierra 150's and headed out to fire a few off. All went well until I pulled the trigger about the fourth or fifth time when I felt like I must have pulled the trigger on a snubby S&W 29. I opened the cylinder and that round had practically fused the primer to the case. I figured that was enough until I figure out what happened. I don't remember all the details of my investigation, but I think it happened to me again on another batch. Always meticulous about every aspect of my handloads I was stumped for a while. I just didn't occur to me to take a micrometer to check the bullets for a while. Finally I did just that and found that I had purchased an entire lot number of bullets where one in five was .001 over size. What I surmised was the the production line had 5 swagers on it one of which was .001 oversized. I ended up miking every single bullet discarding the rest. So to answer your question yes it's critical. At least it is in .357 moderately charged with Blue Dot in my gun.

[SquirrelNuts]

I know it is critical, I was just curious as to where you would have to get to become critical. Sounds like you hit the spot very quickly.

[jim147]

The question with a thousand answers.
I have personally seen a .365 FMJ bullet fired in a .356 barrel without it causing any problems.
It did not have a pressure gauge on it but the next round went off just fine.
I think it would depend on a lot of things. Like powder type, jacket material ect.
You could have one load with an oversized bullet with no problem. Change one thing and kaboom.

jim