Excessive Throat Growth Question

[rscott5028]

id have to dissagree with him hes probably trying to sell a new barrel. the fire cracking on a barrel is usually with in the first 1/2" if you set the barrel back a an inch you have a passed the part of the barrel were the pressures and heat are the highest. from there the bore is usually just fine and will last another 1000 or more rounds. how ever not all barrel contours can be cut back 1"

I tend to agree.

The limiting factors are the amount of firecracking/hardening ahead of the throat and the barrel contour.

It's often best to do your setback before the area ahead of the throat is too far gone. You may still have enough room for a meaningful setback. But if you chase the lands now, you may end without sufficient room for a meaningful setback.

I bet Kirby would have a good idea of what to do after looking at it with a borescope as suggested.

-- richard

 

[Top Cat]

Under these conditions, I think 900 rds is good barrel life.

I'd toss the tube...you got your money's worth. 270 AM is a hi-performance round...if it only lasts 600 rds it's well worth it.

Without actually scoping the barrel, any opinion is just conjecture...so keeping that thought in mind, theoretically, it's possible that a setback, possibly combined with a little throat lapping will give additional service life; but that effort would be something of a gamble, and I wouldn't expect a good outcome to be more than 1/2 of the original life, or 400 rds in your case.

Bottom line is it's only money and it's your call, but I'd rather invest the cost of cutting, threading and re-chambering in a new tube. Consider the extra $400 as a guarantee of a good outcome, and not waste the time of someone qualified to do high level work and your own.

I'm not a fan of half-way measures.

Barrels are cheap...bullets are expensive!

TC

 

[Broz]

Roy, Here is your answer. You don't strike me as the kind of guy that would be satisfied with the rifle knowing it is not 100%. We all know the cost of high velocity, why waste more ammo on her. Order a tube.

Jeff

 

[MagnumManiac]

Not sure how to proper ask but, here goes.

My favorite rig has dropped off in accuracy. I've used the same COAL (Cartridge Base to Ogive) since the barrel was new. Have 900 hot rounds down the old girl.

Guilt edge accuracy has gone.

How much increase in length of throat, from original, is a decent give up and redo point? Or is there something that can be done to squeeze a few more rounds out of her?

As close as I can determine throat has increased 0.047".

I'll give you my take on my own procedure for getting a little longer life out of a worn barrel.

I used to chase the lands, as is a common practice amongst F-Class shooters over here, but I found that if I increased my powder charges until I got the same velocity it was producing before the drop off in accuracy and velocity, and leaving the COAL the same, I could get back within .25-.50 MoA of the original accuracy range.

When chasing the lands, and not changing the powder charge, it was hit and miss whether the rifle would improve on accuracy. I believe this is due to the change in barrel harmonics.
The only way I could get accuracy back to an acceptable level was to start load work up all over again, using more bullets down the tube and worsening the already worn throat.

I also found switching to a double base powder appeared to slow throat erosion. This is going on what I measured and could see with the bore scope.
I know there is a lot of info out there stating that the slower the powder, the more throat erosion that occurs, but I found this not to be correct.
The less gas produced, the less the 'torch effect' has, IMHO.

gun)

 

[royinidaho]

Yo guys!

Many are reading more into this than was intended.

I have no intention of setting the barrel back and reinstalling it on the original action.

Thought of setting it back and rechambering to WSM for a specialty pistol but the replacement barrel is only a #5 and 28" long. I'm really in tune with the longer heavier barrel.

Up to the point where I went back to paper (yesterday) she was still hitting rocks beyond a K. 1 MOA group size is only a half MOA off the mark, max.

All she has to do at this point is hang in there until I complete this repointing thing as there still isn't a high bc 277 bullet that will handle 8 twist @ extreme velocities.

 

[Bart B]

When chasing the lands, and not changing the powder charge, it was hit and miss whether the rifle would improve on accuracy. I believe this is due to the change in barrel harmonics.

Chasing the lands or even opening up the bore and groove diameters one thousandths inch won't change a barrel's fundamental frequency (harmonics are just multiples of that at higher frequencies) more than 1 tenth, maybe 2 tenths of 1 percent. An insignificant amount. Not enough to be physically measured but can only be calculated with math or seen with high speed electronic counters. There's more difference of where the bullets leave in the barrel's low frequency vertical whip cycle with a 5 or 6 fps spread in velocity than what severe throat eroding causes.

 

[Top Cat]

I'd try polishing the throat and first few inches of barrel with JB and see if smoothing that area out tightens the groups. If so, that may have to be a part of the ritual until the tube finally dies. With a tight patch you can feel if it's rough and gets smoother as you work on it.

TC

 

[royinidaho]

I tried all of the above. Negative results

Kept it on the backstop @ 200, barely.

Pulled the barrel and stuck it in a corner.

Packaged the action and new barrel for a trip to Grit down there in Utah land.

Ticks me off as I'm supposed to be woofin' tomorrow and I'm very ill preparred.

Will go and see if I can stumble on to a dumb one anyway.....

 

[bigngreen]

I bet the 375 AM would put a good enough hole in a wolf to slow them down a bit till you get the 270 back gun)