Chamber, throat differences and accuracy

[fatelk]

This is more of a general curiosity question than a specific how-to, so I hope it's OK in Gunsmithing.

Well, I finally alloyed some cerrosafe: http://castboolits.gunloads.com/showthread.php?t=168554

I had to try it out so I made these two casts. The cerrosafe worked well, but now I'm curious about these two barrels.

Some time back, a friend and I each bought an AK74 kit and got them put together. The kits came with US barrels (Yes, I know US barrels in 5.45mm don't have a good reputation.) Neither rifle would shoot better than about 12"/100yds no matter what we tried. Long story short, we spent the money to buy surplus barrels and put them on. They now shoot OK for what they are, not great but far better than before. I did slug the barrels, and they were all the correct bore diameter. I never did figure out what might have been wrong with the US barrels.

One cast is of the US barrel, and the other is of a surplus barrel. As you can see, there is a distinct difference between the throats of the two. Does anyone know what this difference could mean? The top one is the surplus barrel. I notice it has a more gradual throat and longer freebore.

http://i298.photobucket.com/albums/mm274/fatelk/chambercast.jpg

[I'll Make Mine]

Have you checked the length to the throat in the US barrel against the maximum case length and actual length of the rounds you were using? That sharp cut could capture the case mouth of very slightly overlength cases and result in huge pressure excursions from failure to release the bullet properly. Given the variation I see in the mouths of the Russian surplus 7.62x54R ammunition I have on hand, this is likely to happen with any surplus ammo. Even if not dangerous (and if you get a case that's a touch longer, it could be), variations from one shot to the next are always bad for accuracy.

[tomme boy]

The barrels that came on most of the Century built AK's had the wrong barrel on them. They came with a barrel that was 0.224". The 5.45 barrel is supposed to be .222" The bullets were rattling down the barrel of the US barrel. I have seen 2 of these that were completely shot out by 1K rounds. Centry would not honor the mess up.

[fatelk]

Have you checked the length to the throat in the US barrel against the maximum case length and actual length of the rounds you were using?
I hadn't thought of that, but I just carefully measured a few different rounds and there looks to be plenty of room, about the same between the two different chambers.

I slugged the bore also, and it was a hair under .222". I had heard about that Century fiasco, so that was one of the first things I checked. The crown looks fine too.

It doesn't really matter at this point. I'll always wonder what was wrong with it, but I don't plan on using it again any time soon.

[nanuk]

might I add that those are VERY nice chamber casts

very smooth!

[uscra112]

Can't tell from the photo, but look and see if the US barrel's bore and rifling are concentric with the neck. If it's not, it would play merry heck with your accuracy. This is caused by using a reamer with an undersize pilot. Quickest way to eyeball it is to look at the uniformity of the rifling lands.

That surplus barrel appears to have no bullet seat at all, just a forcing cone. That ought to make it worse, not better, but your tests say not so. So I'd be even more curious about the concentricity.

How about the crowns on your US barrels? A cockeyed crown will also ruin your accuracy.

[nanuk]

surplus barrel has a throat like the old BP chambers... or like the 357mag/max rifles.

forcing cones like in pistols