260 throat wear

[Pigdog]

As well as charge weight (ie total energy input) and type (burning rate / flame temperature), and firing rate, there are two more factors

Bullet weight (which is linked to powder burn rate too in many cases). The heavier the bullet, the greater its inertia and the longer it takes to move and gain speed. That extra time exposes the throat / leade / rear end of the rifled barrel to high temperatures and pressures for a longer period.

Pressure - a key factor. Somebody put a post up on a thread about group sizes' relationship to barrel round count on the forum a few months ago (which infuriatingly, I can no longer find). It contains pics of an American Rifleman article of many years ago by a Finnish ordnance officer of a study by that country's army using the 7.62X53R cartridge which is loaded to a maximum somewhere around 45,000 psi. The round counts quoted for the test rifle were staggering - good results well after 10,000 rounds and still going at approaching 20,000 IIRC. Even with chrome-moly steel being much harder than our match barrels' 'stainless', these relatively low pressure rounds apparently induced far less stress than a modern heavily loaded competition 308 Win for instance despite using similar weight charges.

Also, in the UK 'Target Rifle' sling shooting discipline (Fullbore Rifle is the nearest US equivalent) which used ordnance loaded 7.62mm with 145-147gn bullets for many years loaded to lower pressures than 308 Win, barrels easily saw 5,000 rounds plus and many old Bisley hands' practice rifles would go up to 10,000 or even more before rebarrelling. Sometime after 5-6,000 rounds, the elevation spread would steadily increase so that eventually it would cover nearly the entire 2-MOA 5-ring (US 10-ring) so wind reading had to be pretty well perfect for a high score and the V (X) - count would be much lower than for a rifle whose barrel was in perfect condition.

Conversely, I've seen many FTR rifle barrels shot out at under 2,500 rounds - and really shot out as in disastrous performance collapse with very higher pressure loadings in 'Palma' small primer brass. The irony is that with annealing, case life seems to be indefinite for the 'Palma' brass with heavy loads, but barrels can't take those pressures.

Laurie what are your thoughts on what is a max pressure one should stay under in order to ensure reasonable barrel life for example a 6.5x47L is rated to 63KPSI do you think staying under 60Kpsi would ensure a lot better barrel life. Do you think the barrel wear is increasing rapidly in the last 5K or so PSI approaching max?

 

[Laurie]

for example a 6.5x47L is rated to 63KPSI do you think staying under 60Kpsi would ensure a lot better barrel life. Do you think the barrel wear is increasing rapidly in the last 5K or so PSI approaching max?

I would guess so. BUT .... guess is the operative word here. I'm sure there are studies about this, but I can't ever remember seeing one.

The other question of a similar ilk I've had - and can't answer any better either - is:

How does the wear caused by a 'hot barrel-burning powder' (eg Alliant Re17 / Reload Swiss RS60) loaded to moderate pressures compare to that produced by a cooler burning single-based type (eg IMR-4350) in a full-pressure loading?

FWIW, my gut feeling is that as you say, you probably do reduce barrel life by a disproportionate amount with the last 5,000 psi worth of charge in a full-pressure (>60,000 psi) load. The anecdotal evidence seems to support that hypothesis. In 260 and 6.5 Creedmoor, that is around 1.3-1.5gn powder; in 6.5X47L it isn't much more than a single 1.0gn powder weight.

I'll have some opportunity to see for myself with the massive sample of four (Wow!).

Having thrown together a get-started load in a newly rebarrelled 6BR that shoots like a dream but only gives the 107gn SMK 2,700 fps from a 30-inch Benchmark barrel (Viht N150), I've decided to stick with that load and performance level rather than seek the additional 100 fps I'd normally look for.

My short to mid-range 7mm-08 where despite compressed loads of 47gn N160 with the 160gn TMK, pressures should be pretty reasonable (2,830 fps from 31-inch Bartlein).

My Barnard 284 (30-inch Bartlein) which has been fed cool burning Viht N165 almost entirely from new and runs the 175 and 180gn SMKs at 2,830 and 2,850 fps respectively.

Both sevens are now at the 1,600 round mark and still looking very good .....

and finally, my second Barnard F rifle, now being rebarreled with a 30-inch GB Barrels 1 in 9.5 in .300 RSAUM. I have lots of 180/185/190gn JLK and Berger VLDs left over from FTR when those were the state of the art heavier bullets, but which you can hardly give away these days such is the average FTR shooter's infatuation with 200gn Hybrids and the 200.20X bullet. The game plan is to run them only 50-100 fps faster than in 308, so it'll be interesting to see what sort of barrel life this gives.

 

[JBT47]

@Laurie I sent you a pm with a question. I did not want to post it here and derail this thread.