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260AI throat erosion

At 950-1000 round mark I started losing velocity. Measured velocity yesterday with labradar and had 50fps less than my average. Went from 2950 to 2900. So tonight I cleaned then borescoped. I found some throat erosion and cracking. I have only been shooting long range for about 1.5 yrs so this is my first experience with this. My question to you with experience is, what would you do in this situation? I plan on ordering a new barrel. But is their any things I can do in the mean time with this barrel?
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This is a stainless X-Caliber savage barrel 27" long. Chambered in 260AI. Shooting Lapua brass. 140RDF .050" jump. 48.5gr 4831SC. Initial FPS of 2950 now dropped to 2900

Thanks in advance Butch
 
You will find that "generally" a button-rifled barrel will not have the same "life" as a cut-rifled barrel. To make ANY barrel last longer, cleaning properly will greatly aide in longevity. That means either JB Bore Paste or Iosso Bore Paste. If used about every 3-400 rounds, it keeps carbon build up to a minimum, which is the real accuracy destroyer. H-E-A-T causes firecracking. Now your load is 48.5grs of H4831 with a 140 class bullet. T-Rust me, that is a VERY stout load. 2950 out of a 27" barrel is right at the top end! I would SCRUB your bore down with JB and then clean it out. I would bet that you will restore most of your velocity and any accuray loss. However, going down 50f.p.s. seems a bit much. I bet you changed lots of powder>>>correct? H4831 has a very bad habit of not being very consistent from lot-to-lot.
 
@ShootDots Thanks for the info. I did start a new lot of powder. I didnt even think about that.

Help me understand the reasoning behind my load being a stout load? Like I said in first post, I have only been doing this for 1.5 years now and have a ton to learn about internal ballistics. I've been studying external ballistics a bunch but havent made it to internal.

When developing my load I found a really nice high node that is super accurate with no pressure signs. Primers look great, no ejector marks, no heavy bolt lift. It has been very consistant since day one until this.

Thanks again for the info, and keep it coming!!
Butch
 
I have had 4 .260A.I.'s. I never was able to get to 48.0grs of H4831 with 140 class bullets without seeing pressure. Having said that, not getting to 48.0grs was using Large Rifle Primers. I was able to go to 48.7grs with no pressure signs using Lapua Small Rifle Primer "Palma" brass, which is considerably stronger than it's Large Rifle brethren.. However, if you are not seeing any pressure signs, don't worry about it and just go enjoy! Every barrel is different. There are "tight" barrels and "loose" barrels if you will. They are all within spec, but small variations in the diameter of a barrel can lead to premature pressure signs. Your next barrel may very well show signs of pressure at 48.0grs. Just be aware of that..
However, since you know you have a different lot of powder, re-visit your loads to get back to the velocity that gave you the accuracy you once had. Hodgdon powders, especially, Varget and H4831 have a propensity for large variations in the "burn rate". I have seen variations of over 1.0grs of H4831 to achieve the same velocity.
 
I've considered chopping off an inch or so and rechambering? Whats your thoughts?

I own a machine shop and could do all work, would just need to buy a reamer.
 
I've considered chopping off an inch or so and rechambering? Whats your thoughts?

I own a machine shop and could do all work, would just need to buy a reamer.
Mine usually run about 1700 rounds of excellent accuracy. I would NOT do that>>>unless you are into the Long-Range Benchrest discipline.
 
Butch, the fire-cutting you are seeing at the 'lip' of the chamber is to be expected and I would not put any significance in it by itself. How far has your distance to the lands increased from the base measurement; how far have the lands eroded out?. How is the precision...still grouping well in spite of the velocity loss. I agree with Shoot Dots...don't set-back that barrel just yet. I have a few barrels in 6.5 x55 Imp, 6.5 Creedmoor, 6 Creedmoor that are fire cut at the end of the chamber and well firecracked several inches into the bore that shoot just fine. The target determines when to trash my barrels, not the Hawkeye.
 
I havent measured my distance to lands yet. I plan on doing that this evening.



Accuracy is still great. After adjusting my dope for velocity loss, I hit 1300yd 6-10 and 4 in a row Sunday afternoon. (full silhouette) Had to adjust my DOPE 1.5MOA for loss of velocity.
 
Measured my distance to lands today and it is only .006" longer than my original length. (700 rounds ago)

Also found that the last 2 lbs of powder I had used are from the same lot made on the same day.

After cleaning the barrel with sweets I shot again with the same results. Avg FPS of 2900 over 20 shots. Where previous avg was 2950.

Anyone else have some input on where I could be losing velocity?
 
Right at 1,300 rds in one of my 260 AI's. Just used 17.8 gr. Rel 22 with 140's at 2820 fps. Looks fine in borescope with just a little bit of fire cracking. . Shoots and kills perfectly.

The other one I have shoots 130's with H4831 SC. 48.5 gr at 4831fps. No clue about round count as I found an old barrel and am just hunting with it. Super shooting one it is.
 
Right at 1,300 rds in one of my 260 AI's. Just used 17.8 gr. Rel 22 with 140's at 2820 fps. Looks fine in borescope with just a little bit of fire cracking. . Shoots and kills perfectly.

The other one I have shoots 130's with H4831 SC. 48.5 gr at 4831fps. No clue about round count as I found an old barrel and am just hunting with it. Super shooting one it is.

17.8 grain or 47.8? How far has your throat moved? Did you have to chase the lands?
 

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