Bullet diameter vs bore diameter?

[lee1982]

I've been shooting sierra 123s in my 6.5 creedmoor with excellent results, and recently tried some hornady 129 ssts but haven't been able to get them to group at all. The diameter of the sierra 123s are .26410, and the hornady 129s are .26380-.26385. Does the smaller diameter have anything to do with the poor accuracy? I believe the barrel has a .2640 groove diameter.

For what it's worth I have been able to get some 'ok' velocity standard deviation, but the groups on paper are awful.

 

[Jason Boersma]

Skinny bullets can be just like fat bullets your barrel might not shoot them good no matter what you do, if they don't shoot good don't shoot them.

 

[Dusty Stevens]

What did you use to measure that and what temp?

 

[lee1982]

Mitutoyo micrometer at 72F.

 

[jcampbellsmith]

Mitutoyo micrometer at 72F.

Using the micrometer directly on the barrel bore or on a lead slug you had forced through the barrel? Regards JCS

 

[lee1982]

I didn't measure the barrel diameter, just the bullets. It's a standard 6.5mm barrel, so I'm only presuming it is .2640.

 

[pogson]

~0.0002 doesn't seem particularly loose to me. I'd suspect that the muzzle-velocities were a bit different more due to the weight of bullet so you were nearer a node in barrel vibration with one than the other. With a gross misfit, you could get stripping at high pressures/velocities but I don't think you are in that situation. Because one bullet is FB and the other BT, you will have different bearing surfaces and volumes behind the bullet. Both can affect velocities on top of the effect of weight. Some loading manuals show greater than 100 ft/s difference in velocities with bullets of those weights.

Did you tune both loads by trying a range of powder-charges? If one shoots well and the other doesn't it could be you are a grain or two off for the heavier bullet. You might also play with the seating depth.

Normally, being at a node gives you more freedom in charges/velocities for accuracy. It could be that both bullets could do better with some adjustment in charge.

Another possibility is that the shape of the bullets might be different so you are getting a big enough variation in seating depth, position of the base of the bullet in the cartridge and also distance to the rifling. With such light bullets a bit of rattle in the throat might have a larger effect than with heavier/longer bullets. Whenever you change anything in a load, it's worthwhile and safer to rework the charges.

A suggestion, if adjusting the charge/seating depth doesn't help: try slugging the bore with a lead bullet or recovering a jacketed bullet shot into something soft at very long range so you can see the engraving of the rifling. The depth of grooves or other marks on the bullet might give a clue.

 

[dkhunt14]

The best bullets for accuracy have been on the small side for me. You are talking .30799 for 210 Bergers and they broke a bunch of records in 2010. When I get the fat ones they just don't shoot as good. The ones I got this year are .30795 and they shot great. I won all four heavy gun aggregates with them. To me the skinny bullets just have shot better. I don't believe it is the diameter causing it in your gun. Matt

 

[fm1947]

Sierra in my opinion is a far better bullet, that could be why it always is in my case, I use hornady just for plinking around.