260 throat wear

[Slim13]

Just hit the 1000rd mark on my 260. Groups opened up a little so I checked my case base to ogive measurement and it’s .05 longer now. I’ve read about chasing the lands, but now I’m doing it. It’s no big deal I guess. Where does this rank on throat erosion? A lot? A little?

Thanks

 

[Zero333]

0.050" is a lot in 1,000 rounds in the 260 Rem.

What powder are you using ??? H-4350 ???

 

[Slim13]

Yes. 42grains per boom.

 

[Bart B.]

0.050" is a lot in 1,000 rounds in the 260 Rem.

That's exactly the rate that over bore capacity cartridge eroded barrels in match rifles when it was first known as the 6.5x308 in the late 1980's. .001" throat advance per 20 shots fired.

It's 2000 round barrel life was normal for competition. Short range average accuracy starting out at 1/4th MOA degraded about 25% to 5/16ths after 1000 rounds, 50% to 3/8ths MOA at 2000 rounds.

 

[dstoenner]

I have records on my 2 - 260 barrels that I have owned. I measure the throat at 0, 50, 100, 250, 500 then every 500 till the barrel is pulled. My first 260 barrel was a Shillen 26 Sendero profile. It made it to ~2800 rounds and then would not hold a consistent POI after about 40 rounds on a clean barrel. The throat in that barrel was moving at about .025 per 500 rounds. My current barrel is a McGowen 30 Medium Palma profile. It actually is moving at about .032 per 500 rounds. I will have 2000+ on it at the end of this season and I am then moving to a 6x47 as I am not as tolerate of recoil to my shoulder due to some injury.

So what you are seeing is what I saw pretty much in mine. I have heard that when a barrel throat has moved .150 then it is toast. My first would have been close to that. I never measured the final state so I don't know. I had the new barrel, it was the end of season, off it went.

HTH

David

 

[Slim13]

Thank you Bart and David. It sounds like I’m experiencing typical wear. I’d also like to add that what made me check my distance to lands was horizontal group dispersion. I checked the CBTO measurement on my ammunition when I got home and it was to spec. The ammo didn’t change, but the distance to the lands did.

 

[Bart B.]

I have heard that when a barrel throat has moved .150 then it is toast

With some barrels, its about half that.

I think it depends on throat angle and bullet ogive shape. Throat and bullet combination with bigger angles at their contact point will have the throat move less before accuracy suffers some amount. More friction at the contact point.

 

[hunter686]

My current 6.5 SLRI has just gone 700 rounds and the throat has moved 0.011 buring 45.5gr of H4350. Just to give you another reference point.

 

[Twicepop]

IMHO the .260 Rem and it's kin all suffer from the same problem, they're based on the .308 Win case. The .308 is a great round for what it was designed for, to feed and extract easily from military rifles, both semi and full auto. It has quite a bit of body taper and a shallow shoulder angle. The shoulder angle problem was put forth in an article in Precision Shooting several years back. The shallow shoulder angle allows the stream of hot gasses to contact the throat and erode/burn away material from the barrel, in the article it was called the convergence point. Many of the newer cartridges being designed have less body taper and shoulder angles of 30 degrees or higher to reduce the burning and increase case capacity.

 

[JasonT]

IMHO the .260 Rem and it's kin all suffer from the same problem, they're based on the .308 Win case. The .308 is a great round for what it was designed for, to feed and extract easily from military rifles, both semi and full auto. It has quite a bit of body taper and a shallow shoulder angle. The shoulder angle problem was put forth in an article in Precision Shooting several years back. The shallow shoulder angle allows the stream of hot gasses to contact the throat and erode/burn away material from the barrel, in the article it was called the convergence point. Many of the newer cartridges being designed have less body taper and shoulder angles of 30 degrees or higher to reduce the burning and increase case capacity.

Then why do many FT-R rifles often have a 3000 round accurate life? I think it's got more to do with the amount of powder being burned in the small bore area of the 260 vs the 30cal bore of the 308.

Also, the 308 has only ~0.020" of body taper, which equates to about 0.01 per inch. That's not usually considered a lot of body taper as far as I've seen.

Supposedly, Ackley wouldn't even improve the 308 himself because the body taper was already minimal and the neck was already a caliber long. Further, that the gain from improving the 308 is only in trimming frequency (and therefore slightly improved brass life).

 

[Bart B.]

Then why do many FT-R rifles often have a 3000 round accurate life? I

The objectives, conditions and standards were different for those establishing that round count. A high master will rebarrel his 260 at 2000 rounds; a marksman at 4000 rounds.

Most important is the percentage accuracy degrades that's the worn out point. Sierra's was 50% for their match bullets starting out at 1/4th MOA average with a new test barrel; worn out at 3/8ths MOA.

 

[Laurie]

What will be interesting to see is how Creedmoor and 260 barrel lives compare now that the 'keener types' have shifted to small primer brass from Peterson and Lapua in the former. While both cartridges used large primer brass and the Creedmoor gained a reputation for expanding case-heads and primer pockets in no time at all with the original soft Hornady product (sometimes on the single firing of a factory cartridge even), both were being run at 57-58,000 psi by most handloaders, as were factory cartridges.

Now that people can load their Creedmoors to 60,000 psi plus (and I'd bet nearer 65,000 in a lot of loads) with SR brass, I suspect that it'll produce shorter barrel life than milder loaded 260s despite the smaller case and powder charge, despite the longer neck, and despite the 30-deg shoulder angle allegedly giving a TPI position within the case-neck.

I'd be happy to be proven wrong on this, but time will tell one way or the other.

 

[ebb]

Ditch the H4350 and try something that don't burn so hot. I don't know if you can get enough H1000 in the case to get FPS desired, if not try VV160 or VV165.

 

[Twicepop]

Then why do many FT-R rifles often have a 3000 round accurate life? I think it's got more to do with the amount of powder being burned in the small bore area of the 260 vs the 30cal bore of the 308.

Also, the 308 has only ~0.020" of body taper, which equates to about 0.01 per inch. That's not usually considered a lot of body taper as far as I've seen.

Supposedly, Ackley wouldn't even improve the 308 himself because the body taper was already minimal and the neck was already a caliber long. Further, that the gain from improving the 308 is only in trimming frequency (and therefore slightly improved brass life).

I knew of the "long life" of some .308 barrels when I posted that. I've heard of some of them going as much as 5K rounds before being replaced. The main gist of what I posted was about the shallow shoulder angle, a 20 degree angle puts the blow-torch effect of the hot stream of gasses out ahead of the brass case and directly into the throat. Reducing the bore size intensifies the effect, like in the .260 and the .243 a known throat burner.

 

[DaveTooley]

Initially throat erosion will vary depending on bullet shape and lead angle in the chamber. SAAMI specs for the 260 has a 3 degree lead per side. That coupled with a more tradition tangent shaped bullet will not show wear as quickly. The other extreme- most of us use a 1 1/2 degree lead per side. Couple that with secant ogive bullets (low drag) and just a slight amount of erosion will rapidly move the engagement point further forward.
I don't think there will be a nickel's worth of difference in barrel life between the 260 and the 6.5CM .

 

[Laurie]

Having got you on the Topic Mr Tooley, you might be able to throw light on one puzzle with this pair for me. I notice from the PT&G reamer drawings that 260, 6.5 Creedmoor and 6.5X47L have 0.2645" dia. throat sections, a mere half thou' above the bullet nominal diameter. (I have several hundred 130gn Berger AR-Hybrids that mike marginally above 0.2645" !!) I had trouble with some bullets not clearing the throat cleanly on a couple of rifles. The 6.5X55mm SE has two or three thou clearance.

Why so tight?

 

[DaveTooley]

Just a trend that comes back and bites some in the butt. I have long since stopped using freebores that tight. They violate rule #1 Don't do anything to make the phone ring. All the pluses and minuses tolerances add up to more than .0005"
Personally I like the freebore +.0012" over nominal bullet diameter. Everything, barrel, bullets, chamber etc match up better that way.

 

[dstoenner]

They violate rule #1 Don't do anything to make the phone ring.

That was funny but I understand. If I were a gunsmith I would take that approach also.

 

[JasonT]

I knew of the "long life" of some .308 barrels when I posted that. I've heard of some of them going as much as 5K rounds before being replaced. The main gist of what I posted was about the shallow shoulder angle, a 20 degree angle puts the blow-torch effect of the hot stream of gasses out ahead of the brass case and directly into the throat. Reducing the bore size intensifies the effect, like in the .260 and the .243 a known throat burner.

I've often heard that hypothesis regarding the shoulder angle and the direction of explosive (or more appropriately conflagrative) gases.

Setting aside any human involvement, I think that the biggest contributor to barrel life is the amount of powder burned in a specified bore size.

Then the next would be the type of powder, where slow powders would continue to burn longer as the bullet progresses down the barrel, effectively continuing a full intensity explosion down 10" of a bore instead of fast powders that are completely burned before the bullet is 3" down the bore.

That said, I've seen two identical big-name barrels in HV contour, one in 308 Win, shot by somebody who'd fire every 10 seconds; and the other barrel in 338 WSM shot by somebody who waited a full 60 seconds between shots. The 308 was blown out (more than 0.5moa groups) by 1400 rounds and the 338 WSM was good until 1800 rounds.

I think the person behind the trigger is one of the biggest contributors to barrel life.

 

[Laurie]

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.