FNSPR chrome lined bore problems - help required

[ex-skydiver]

Dear All,

I've had a .308 FN USA rifle since 2008. I believe a new barrel is required since the gun has lost accuracy noticeably. Chrome lined barrels are supposed to last longer than non-chromed lined barrels so with only about 2,500 rounds through this gun I'm beginning to think this is not always the case. Following is information and a picture of the problem is attached, so if you have any comments or ideas then let me know. I suspect a re-barrel is required so if anyone has re-barreled an FN SPR I'd be interested in the details. I have contacted FN but their response has been zero.

Rifle details:
First owner: 300 x Lapua factory ammunition (Scenar 168gr).
My ownership: 800 x Hand loads VVN140, Lapua Scenar 168. Lapua brass
1300 x Handloads VVN550, Berger 185VLD / 185Match boat tails, Lapua brass
100 x surplus ammunition since I purchased the gun.
Total: 2,500rnds

Accuracy was around the 0.5MoA to 1.0MoA for the first 2200 shots. The accuracy then became between 1.0MoA and 3MoA.

Throat erosion became apparent at around the 2200 (total) shot mark as the VLD rounds suddenly lost accuracy (without any change in loading powder or bullet etc.) so I had to advance the bullet seating forward 6 thou'. At this time I also noticed a crazing/cracking/roughness on the lands at the throat that extended about 5mm. This cracking/crazing has increased in length towards the muzzle to a length of about 10 - 15mm in the following 300 shots until now. I then developed a load for Berger MBT heads which are less prone to bullet seating depths. This initially seemed to work well, but the accuracy became inconsistent again to about 3MoA. Attached is picture that does show this potential erosion/cracking but I admit it isn't too clear - as I don't wish to remove the barrel from the gun unless I re-barrel.


I have also used a boroscope to look at this as I initially thought it was copper or stubborn carbon deposits but it appeared again to be a "crazing" pattern. I have my doubts that the throat has become eroded enough to cause a problem because of the chrome lining and the low number of rounds through it so I'm struggling to find a cause for the loss in accuracy. However, I do know that when the chrome lining at the lands does eventually become worn through that accuracy suddenly and significantly changes for the worst.

Any ideas?

Many thanks

Jo

[dromia]

Everything else checked, screws tight, bedding OK, Scope mounting OK?

[Dangermouse]

I can't help but wonder if the double based Viht 550 is the source of the barrel wear.

DM

[The Gun Pimp]

I (and others) have ditched 308 barrels with a little over a 1000 rounds down 'em after using double-base powder. As Dangermouse suggests, the N550 is a possible cause.

[meles meles]

We were sitting in a ditch one day, minding our own business, when out of the blue a hot steel tube landed on our head !

*snarls*

[ex-skydiver]

Many thanks for the messages gents. Yep, everything else checked, screws tight, bedding and scope mounting all ok - well worth mentioning as it is usually the most obvious that is overlooked.

Since the primary advantage of the chrome lining is wear resistance I hadn't thought about the use of double based powders. I've not heard of this before, so can someone say what these powders contain or do that attacks the chrome lining (or any other barrel metal) in such an aggressive way?

The absolute silence from FN USA and FN Herstal about this anomaly on the lands does make me think that something isn't right since it would contradict the claim maintaining 0.5MoA accuracy up to 10,000rnds as conducted for the FBI trials in 2003.

[waterford103]

Double base powders contain nitroglycerine as opposed to single base which are nitrocellulose only .The nitroglycerine elevates temperatures and pressure thus causing greater throat erosion .

Barrels chromed for military use use single base powder and do have a long life.

PS cracking in hard chrome is potentially dangerous as the hard chrome is bonded at a molecular level and the cracks may propagate

beyond the chrome / steel substrate and cause barrels to split.

[ex-skydiver]

Many thanks Waterford103,

I chose N550 for is slow burn and lower peak pressures, so I would assume it's the potential elevated temperature that has or possibly caused the throat erosion here. The FN SPR was purchased for the FBI hostage negotiation teams (I assume when the negotiations were nearing Krispy Kremes closing time) so I would think the potential ammunition fed to in-service guns wasn't likely to be standard military issue. That said I guess FN would have used the least aggressive/lowest pressure ammunition for the tests which could have been military grade.

The appearance of the problem looks like scratches on the throat but these could be many small cracks, so either way I think a re-barrel is required.

Many thanks to all who responded.