Somebody check my theory...

[brmfan]

I have a Remington 5R that I shoot 178 AMAXs on top of 45.0 of Varget, Lapua brass, and 210M primers. I have always gotten ~.5 MOA at 100 yards with this load and it's been pretty decent at 1000 yards as well. Anyway, I recently had the lugs lapped whereas before I had about 25% contact on one and ~ 30% on the other (from the factory). Now I have almost 100% contact on both, and when I tried the same load again (post-lapping) I had several consecutive 5 round groups go less than .25MOA just yesterday.

Anyway, my guess is that the higher pressure of the increased load caused more bolt flex due to uneven lug contact and magnified the shot-to-shot POI variance. Now, with almost 100% lug contact the blot flex should be significantly reduced (since the chamber pressure force is now more linear) and as such the higher pressure loads will now (hopefully) display the same level of accuracy as those with lower pressure.

Does this sound right???

[Scorch]

Yes, pretty much.

[QBall45]

Your theory is sound.


Now, go prove it. Prove your theory to yourself. It should make an interesting experiment. Please post your findings when your done.

[5R milspec]

just wanted to ask.what COAL are you useing for that load?I have a 5R and been wanting to try the 178g A-max.for me I load with a 175g smk and a 168g smk.and both at 2.800. John

[brmfan]

5R: Here is the load for this rifle that shoots ~.25 MOA all day long

Fire formed Lapua brass (neck sized only with Lee collet sizer) - I scrub the inside of the case necks with a stiff nylon brush before sizing.
44.5 Varget
178 AMAX (although the 168's also do about as well with the same load, but they're not for long range)
Federal 210 primers (although many folks like the CCI BR2s)
2.007 case trim
2.830 COAL for 178 AMAX (These rifles have really long throats. When not loading for a competition I seat the 168s at 2.810 to fit in the AICS magazine)

* For some reason my rifle HATES SMKs but loves AMAXs
** I replaced the XPro trigger with a Rifle Basix
*** It's in an AICS 2.0 stock, although the HS worked well. I just prefer the AI.
**** The lugs were lapped

[Unclenick]

Sounds like you need to buy a copy of Harold Vaughn's book, Rifle Accuracy Facts. Precision Shooting sells it on their site. It discusses and illustrates measurements of barrel and receiver deflection moments and their effect on precision of the shot placement.

Try the 175 grain SMK if your gun doesn't like the 168. The 168 is a late 1950's tangent ogive design for 300m International match shooting that, as a Sierra tech explained to me, it was just their good luck it also flies well to 600 yards and became so universally popular. At about 700 yards from the .308, it falls into the transonic velocity range and becomes unstable and tumbles, so 600 is its practical limit. The 175 is a secant ogive design worked out with the military in the late 80's for long range sniping and has proven itself to 1200 yards. It remains stable through the transonic range.

The benchresters I know all claim Berger bullets are more consistent than either Hornady or Sierra, so that's another avenue for exploration if you get bored with what you have?

Also take a look at Dan Newberry's OCW site. Even if you aren't after a universal load, the round robin method he uses to find the powder charge is a superior substitute for the old Audette ladder, IMHO. Every bullet needs its own load, as a rule.

Your experience with the lug lapping is a reminder that the world's best pet load can't correct fundamental mechanical problems in a rifle. One of the chapter authors in the Precision Shooting Reloading Guide stated that he disliked the concept of the pet load for that reason, since it seemed to make out that the load bears all responsibility for good groups. His comment was that it was commonly the case with commercial grade rifles that if they haven't had their lugs lapped, muzzle re-crowned, action bedded, and (in some cases) barrel firelapped, you don't really have any way to find out what they are capable of with the best ammo? I'm not recommending you rework those other elements on your gun, since it seems to be shooting well, but it is a basic list that should be considered by all owners of non-custom built or smithed rifles.