Case neck problem!

[DaveMarine1]

I have been doing load developement with Lapua 308Win brass. This is new brass, I FL and neck size it and then run it through a Sinclair 308 expander mandrel, then I debur inside and outside. I prime it, powder it, and seat the 168gr. bullet .015 off the lands. I am having trouble finding some consistancy in my groups. The FPS, and SD seem to be for the most part consistant at 2670-2690fps. The chamber was cut to match grade and also the crown is new. Barrell was broke in and cleaned about 20rds ago so I dont suspect fouling. What I see on the case necks after firing is that the carbon will go down the neck about 3/4 the way for about 80% of the neck circumferance and the last 20% it is about 1/4 of the way down the neck. Is this telling me that the chamber is not sealing and allowing pressure to blow back? I see this with no matter what charge of powder I use. Also, after shooting, the neck seems to be out of round, almost like a flat spot about 1/16 of an inch or less. What are your guys' thoughs on this? I appreciate your time.

David

 

[Shynloco]

David,
IMHO, the carbon deposit you describe on the necks suggests the throat on your barrel was cut at one size and not matched to your casing necks. In short, the reamer wasn't made to case specific sizing. I get the same thing in my 6br, .308, 6.5 and .223 because the reamers weren't case specific. Having said that, I'm not exactly sure what you mean by "consistency". I'm "ASSUMING" that you aren't happy with the groups. If that's the case, I believe though you have good crony readings, you still have more work to do on the loads. My best groups in my .308 Match Grade Barrel come from(using 168 Sierra HPBT) either Varget or RL15 in the 42 gr range. And I mean 5 shot groups touching even with the carbon deposits on the necks. And to be perfectly honest, I think though the groups may touch, the additional variable or "inconsistency" is on me because of my aging eyes and not the fault of he rifle, reamer or any other variable. Keep at it OR remove the barrel and have it recut using a custom reamer that is case/size specific. Just something to consider. Good Luck!

P.S. As an afterthought, have you measured the thickness of the necks on your casings? Even with Lapua brass, you can get differences occasionally in neck thicknesses depending on when the brass was made and if it was later mixed with another lot of brass when reloaded.

Alex

 

[dmickey]

Twenty shots will hardly be enough to "break in" a barrel! In my 40XB Rangemasters, I find that it takes over one hundred shots to notice a definite improvement in the group size (not that it's bad to start with!) ;D

 

[DaveMarine1]

I see i could have been misunderstood. The barrell was broken in with more than 20 rounds. What i meant was the barrel was cleaned 20 rounds ago. Sorry for the confusion.

 

[tenring]

The expander mandrel should be used before you neck size, not after.