Why does my rifle shoot better fouled with copper?

[BluegrassDan]

My Kimber 84M in .308 Win shoots better the more rounds I put through it. It generally shoots best after at least 60 rounds, and then shoots great for at least another 100-200 rounds. I can get sub MOA at this point. I've shot a couple of .3" groups at 100, and most are between .5" and 1"

The gun fouls with copper like crazy! As soon as I clean the copper out entirely, the groups open up to about 1.5"-2" eventually settling down after a few boxes of ammo. I've tried the whole break-in, but it didn't fix the fouling.

So, what do you experienced guys think? Is there a reason for all of this? Does it make sense somehow?

Thanks in advance!

 

[Got_Lead?]

I don't understand it either, why some barrels shoot well fouled, and some like to be clean. I have a Ruger international in .308 that metal fouls terribly, it takes hours to clean it with sweets after firing just 20 rounds. I haven't worked up a load for it, so I don't really know what affect the fouling has on this barrel. My Remington 700 in '06 doesn't foul at all, and it's very accurate, same for my AR-15, which has a stainless barrel.

My revolvers lead terribly in the leades, however, they will still consistantly shoot 1 1/2" groups at 25 yards, go figure.

I am learning to not get worked up over a bit of fouling in a barrel after a shooting session. I just wipe down the gun, and apply rust prevention. Scrubbing to bare metal in the bore seems to be over rated.

Well, the good news is that you have figured out what makes your rifle shoot, maybe you can get a routine of only cleaning just so much of the fouling out, say a few brush strokes with hoppies or clp. I wish I could lend some more analytical insight, but it bewilders me too. You are not alone, so I guess whatever works is good to go.

Good shooting.

 

[M1key]

The copper fouling probably smooths out the rough bore.

Just guessing...

M

 

[P-32]

As already stated some barrels are just that way.:banghead:

 

[Got_Lead?]

Bluegrassdan:

Have you experimented with your loads to see if you can improve your rifle's accuracy when the bore is clean.

There is probably more bullet friction in a fouled bore (just guessing), this could affect pressures and powder burning. Your rifle might just prefer a slight adjustment in powder charge to bring her back into the accuracy sweet spot while the bore is clean.

I can't speak for rifles right now, because I haven't done extensive chrono work with them yet, but in my revolvers I get almost 100 fps gain after building a bit of fouling from firing 20 or so rounds out of a spankin clean barrel.

I wonder if the difference in friction accounts for slight changes to your internal ballistics, and you could recover your "clean barrel" accuracy with a small powder adjustment? I'd give 4 bits for knowing if it would work.

 

[sansone]

fouled bores tighten-up the fit, increase pressure slightly, smooth-over tooling burrs, etc.
I work up loads cleaning every 4-10 shots to avoid confusing results.
My first shot from a freshly cleaned bore never hits in the same spot as the next 5.
you could just leave it alone and shoot it fouled if the gun likes it that way

 

[jmr40]

I've never shot a rifle that didn't shoot better after a few rounds had been through the bore. I only clean mine after accuracy starts to fall off, which can be several hundred rounds with some rifles.

 

[Zak Smith]

I think your question contains your answer. If it fouls like crazy, then the rate of change of the bore condition is greatest when it starts clean and you put the first few tens of rounds through it.