Chamber/Throat scapes coating off

[2manyToys]

I pulled some powder coated bollits out of my test barrel (filled with rubber mulch) and found that they had some of the coating scraped off closer to the base of the boolit, well below the case mouth. It's not scraped all the way around the boolit, only about 20-30° around the diameter. This is from two different firearms - a Glock with an aftermarket barrel and a Ruger PC 9 carbine.

I size to .357, bell the case mouths, crimp separate with a taper crimper only enough to take the bell out. I have pulled boolits with an inertia puller and verified that the coating isn't being damaged in the loading process. Using a bore scope, I can see the coating that's left behind at the sharp edge at the forward end of the chamber/throat interface.

Can this only be fixed by having the barrel thoated? I don't think that the Ruger is Melonited, but the aftermarket Glock barrel is, making the surface very hard. I thought about trying to put a small radius in there with a cratex point, but concerned about maintaing concentricity. Any ideas?

[Iron Whittler]

Send them to Dough Guy. He is a magician with these kind of problems. :Fire:

[Taterhead]

For the Glock, try .356. My G17 runs PC just fine. If that doesn't work, I'd look to the PC and the curing process.

EDITED TO ADD.

Try the stock Glock barrel. It has a gentler transition.

[Conditor22]

IF this happens in 2 guns I'd check and see if your mold but mostly the sizing die might be out of round.

measure a sized boolit and see what you get

[Dragonheart]

Some good photos would help. but as already mentioned try sizing .356". I have a lot of 9mm's, Glock's, Wather's, Sig's, HK, S&W, Springfield, et. etc. I have water trapped fired slugs from all the barrels and all these barrels will seal with 356".

I have found no benefit in shooting oversized PC bullets because in my limited Ransom Rest testing identical loads with the only variable the bullet size, my group sizes increased as the bullet size increased. For years I have listened to ole timers say. "I get better accuracy with 2-3 thousands oversize bullets". When i was a green kid I believed them. But now I am an ole timer and I can say from my test results, "you are going to have to prove it to me under real test conditions, not shooting offhand at a tree stump, because my accuracy test results show exactly the opposite". It stands to reason, once the approximately .004" rifling groove is filled and the barrel sealed then excess pressure is going to build as the barrel is forced to swage down the excess bullet mass, so how could accuracy improve?

My first thought would be cutting the coating when seating the bullet; your pulled bullets ruled that out. But I would not have thought .357" would completely strip off a properly coated and cured PC. So I would suggest doing a scrape test on your bullets to verify that the coating is hard. The smash test will show if there is a good PC bond. An undercured coating could explain the stripping and buildup.

All cast bullets are out of round when they come out of the mold and sizing only makes the surface area where the die touches round. The remainder of the bullet is still lopsided. With this in mind could you be seating your bullets too for out for the barrel? Excess OAL could also explain the chamber buildup, if the bullet is damaged when chambered.

I check my barrels with every bullet type that I use to determine the OAL for that barrel/bullet combination. Once measured I put the information in a spreadsheet with load data. This is easy to do:

First for 9mm Luger find a sized case the optimum length of .750". The 9mm headspaces off the case mouth as you probably know. With a dremel tool and a fine cutting disk make two cuts on opposite sides of the case starting at the case mouth and going straight down slightly past the seating depth of the bullet.
273243273244

Place a bullet just in in the mouth of the case where it is held firm. Then with the gun barrel removed press the case firmly into the chamber. You may need the aid of a tool like the flat blade of a small screwdriver or other, to gently remove the case. With a micrometer measure the OAL. Do this a couple of times until you feel confident you have a good measure. This measurement is where that bullet design engages the rifling on that barrel. Adjust your seating die so you have a few thousands of bullet jump for the best accuracy in that gun, or find an OAL that works for all your guns. Don't be surprised to find that identical firearms have different OAL's.

Maybe one of these suggestion will find the answer as I really doubt two handguns would both have a problem.

[DougGuy]

Sounds like the same fitment problems anyone would have with a barrel with a small throat or no throat, and cast boolits. Add PC to the mix and now you have an even larger cast boolit, so now you have even more interference in the throat.

Have the barrel(s) throated with .3585" freebore, long enough that you can seat out ever how long you want to as long as it will cycle through the magazine, and be done with it. Assemble loads with .358" boolits, remove the barrel, check them for plunk fit in the barrel, put the gun back together and go enjoy shooting it.

[popper]

When the powder goes off to bullet gets sized by the barrel and metal moves to the rear. It piles up on the base and gets stripped off by the edge of the bore. RN is worst depending on top punch, seating actually straight is uncommon. If expander is not long enough, base is squished compounding problem. a large chamber will allow the case to sit crooked and edge will scrape one side. Bad feed from mags will strip some drive band when chambering - that will continue to strip when fired. If accuracy is acceptable, forget it. Your target doesn't care. Generally only a problem when it interferes with full chambering.