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I recently bought two boxes of SMK 120 #1725 to try since I had never used them. When I looked at the suggested O.A.L. of 2.220" I checked my barrel with my O.A.L. gauge to see what the max O.A.L was and it checked out at 2.305" that works out about 1.787"-1.790" from the base to the Ogive. My 123 gr Amax now check out at 2.350" O.A.L. (1.765" now -vs- 1.725" then) which used to be around 2.310". From the measurements it seems like my throat has eroded quite a bit for 1510 rounds down the pipe. I have been using mostly 8208 and Lever Revolution.
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How closely have you tracked accuracy while sending those rounds downrange?
How much rapid fire was involved?
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Not every bullet is the same at the ogive and even different runs of the same bullet will have different ogive measurements . So if you want to keep track of throat erosion you would have to use a certain bullet and put it away to use the next time you measure.
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JASmith Wrote:How closely have you tracked accuracy while sending those rounds downrange?
How much rapid fire was involved?
I allow about 5 seconds between shots. I use 5 rounds at a time. The accuracy has gotten steadily worse. I cannot shoot a decent group at 100 yards with it. That was the reason for going to the SMK 120. If it won't shoot a decent group with them I am considering getting a Lilja barrel.
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One man's "decent" group is anything less than 0.3 moa while some are very happy with 1.5 moa at 100 yards.
What size groups did you enjoy during the first 500 sots and what were they during the mos recent 500 Shots?
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1500 shots should not typically cause that much erosion, and certainly should not see a degradation in accuracy. I went 5000 plus rounds before I began to suspect my accuracy had dropped off, and it took another 1000 before I was convinced.
Something else is most likely causing the accuracy issue. Check your gas key and see if its loose, check your scope rings, check your scope bases. Swap scopes if you can. I've seen 2 high end scopes with broken internals the last two visits I've made to the range. (I know, I know everyone says the same things! But I was at a range Saturday where a 3 gun match was running when one of the competitors walked up and asked if he could try to zero his carbine. Its was shooting in another neighborhood, and so I started trying to help him figure it out. When I started checking ring screws, some of them weren't even finger tight!)
Start with those and then get pickier and pickier about stuff until you find it, but I have a hard time believing that its related to throat erosion at this point.
You seek escape from pain. We seek the achievement of happiness. You exist for the sake of avoiding punishment. We exist for the sake of earning rewards. Threats will not make us function; fear is not our incentive. It is not death that we wish to avoid, but life that we wish to live. - John Galt
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Throat erosion in a 6.5 Grendel running at or under 50,000psi?
What loads are you using for charge weight?
Also, the Stoney Point (Hornady) gauges need to be fully seated into the chamber to get a reliable reading, otherwise your COL will come out with the slop factored into the number.
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JASmith Wrote:One man's "decent" group is anything less than 0.3 moa while some are very happy with 1.5 moa at 100 yards.
What size groups did you enjoy during the first 500 sots and what were they during the mos recent 500 Shots?
The groups now are around 2 MOA. The first 500 rounds would yield sub MOA. Believe me when I say I have been scratching my head checking everything I can think of.
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LRRPF52 Wrote:Throat erosion in a 6.5 Grendel running at or under 50,000psi?
What loads are you using for charge weight?
Also, the Stoney Point (Hornady) gauges need to be fully seated into the chamber to get a reliable reading, otherwise your COL will come out with the slop factored into the number.
With the 123 grain Nosler I used 27.0 grains of XBR for an average of 2382 f.p.s. With the 108 grain Lapua I used 28.3 grains of XBR for an average velocity of 2591 f.p.s. to 31.0 grains of XBR for an average of 2624 f.p.s. Nothing that I would consider a "hot" load. I have shot very few loads that indicated "excess" pressure. With the 123 grain Amax I used 29.9 grains of Lever Revolution for an an average of 2478 f.p.s.
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maverick5582 Wrote:With the 123 grain Nosler I used 27.0 grains of XBR for an average of 2382 f.p.s. With the 108 grain Lapua I used 28.3 grains of XBR for an average velocity of 2591 f.p.s. to 31.0 grains of XBR for an average of 2624 f.p.s. Nothing that I would consider a "hot" load. I have shot very few loads that indicated "excess" pressure. With the 123 grain Amax I used 29.9 grains of Lever Revolution for an an average of 2478 f.p.s.
You're about 2.2 grains higher than the Hodgdon's max load for the 107gr SMK, and the 108gr Scenar is longer than that one. There's your culprit if you do in fact have throat erosion. Pressures are probably in excess of 60,000psi easily.
What barrel length and bolt are you using? What barrel?
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04-06-2015, 11:38 PM
(This post was last modified: 04-06-2015, 11:52 PM by maverick5582.)
LRRPF52 Wrote:You're about 2.2 grains higher than the Hodgdon's max load for the 107gr SMK, and the 108gr Scenar is longer than that one. There's your culprit if you do in fact have throat erosion. Pressures are probably in excess of 60,000psi easily.
What barrel length and bolt are you using? What barrel?
24" Green Mountain barrel and Alexander Arms bolt. That would be a misprint it was 31.1 grains of LVR at 2611 fps with the 123 Nosler. I had to go back to my original load sheet and not the log book, my mistake.
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The 108 Lapua load was a max of 28.3 grains of XBR at an average of 2591 f.p.s. I keep all the target and load sheets in a file. Better to go by them than the log book entries. My bad.
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arnie Wrote:Maybe the crown ?
The barrel has a 11 degree target crown. The pattern left by the gases expelling the bullet has left the traditional star pattern which would suggest that there is no problem there.
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I admit that "throat erosion" is the last of a long list of things that that I have considered being the culprit here. The only time that I have had a problem with it was with my 6.5-284 Norma bolt gun. Shooting the 140 grain pills at 2900 f.p.s got the throat in 1200 rounds. With the AR I have tried to keep all the loads under 52,000 PSI I have two bolts and a spare carrier. I will switch out the bolt and carrier next time at the range. Other than that I am at a loss to explain why the rifle will not shoot under a 2 MOA group with all the different loads I have tried.
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This is a crazy idea, but could it be the meterials used to make the barrel? We has a serious issue with an Ed Brown 1911 once, long story but bottom line I would never ever own an Ed Brown and they used cheap materials that caused this 1911 to fail! The material was too soft... could this have caused the erosion if your loads were where you say they were?
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maverick, Have you given your barrel a good cleaning not just for powder fouling but also for copper fouling using a copper removing solvent? Copper fouling will kill the accuracy of the best barrels.
pds
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pds Wrote:maverick, Have you given your barrel a good cleaning not just for powder fouling but also for copper fouling using a copper removing solvent? Copper fouling will kill the accuracy of the best barrels.
pds
I gave the barrel a thorough cleaning today and then checked for copper fouling, no blue tint at all. Rechecked all O.A.L dimensions with Stoney Point gauge, no change there either. Life goes on.
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