How long can I expect a chrome lined, Bushmaster barrel to last in semi-auto fire?  I'm thinking of buying one with 6-8,000 rounds fired.  Can I expect another 20,000 rounds before I need to replace it?

Barrel Burnout

This should help you

I don't know but I see a lot of Garands, M1 carabines, etc. dated back to 194x that still shoot fine

Not to hijack here, but what is the most effective way to clense the barrel of all the copper after oh say 12k rounds. And should it be good for another 12k after that?

MTBF for the M4 is 9600 rounds... barrel life is dependent on heat probably more than anything (granted, most barrels are killed by users that just treat them wrong) so the difference between slow fire, rapid fire, substained fire and full-auto fire has a large impact on this.

The barrels that last to 20K are out there, the ones that I know of are made from grades of steel that are not common or are on guns that live an easy life of casual shooting and proper care.

Without any knowledge of how a barrel was used and cared for, I would consider a barrel with 8K to be nearing the end of its service life... but this also depends on what you expect as far as accuracy goes? You may want to gauge the barrel and check it that way?

Quoted: Not to hijack here, but what is the most effective way to clense the barrel of all the copper after oh say 12k rounds. And should it be good for another 12k after that?

While CLP is good enough for expediate cleaning for a job like this I'd use a serious bore cleaner like Sweets 7.62, (be sure and follow the directions as this is not some mamby-pamby bore cleaner but serious stuff).

FWIW, I use this to clean my ARs and rinse the Sweets out, (prior to lubing w/ CLP) using the non-methanol SuperTech 2000 carb cleaner sold at Walmart, (works well).

Mike

E.T.A - FWIW, as to the original question, I'm with PASS crowd, (too many rounds down the tube, under un-known conditions....)