Freebore Diameter, Bullet Diameter, and accuracy

[Stumpy1]

So, I have this 6.5 that just doesn't seem to want to shoot despite best attempts to get it dialed in.

When I measure the bullet shank diameter, I get 0.2639" (and yes, I can properly read a micrometer), these are Berger 144's.

Measuring the reamer freebore diameter, I get 0.2649".

So, I have ~0.001", clearance between bullet and freebore -too much?

Secondly, and the point of this question, when I measure every other bullet in my inventory, regardless of caliber, they mic out at slightly ABOVE caliber diameter -like 0.3083-4" for .308, 0.2243-4 for .223. So, are the 144's I am using undersize at 0.2639?

Ideas wanted!

Thanks in advance,

Frank

 

[Dusty Stevens]

Bad bullets get thru more than you can imagine. Try another box and see what you get

 

[DaveTooley]

I have all my reamers made with the FB +.0007" to .0012" over nominal bullet diameter. People think in absolute numbers. There are manufacturing tolerances that can and will stack up as well as reamer wear. The last thing you want is the remnants of a land coming back to neck. I have OEM accounts that ask what 308 reamer I use because they can't get there work to shoot as good as mine. I have used a SAAMI reamer for 14 years. That has a .310" diameter FB. Concentricity and the marriage between the barrel and bullet have the most influence.

 

[wkdickinson]

I had a similar experience with 6.5mm Berger 140 Hybrids a couple years ago. Unfortunately, I can't find the data. But as I recall, I measured about 20 out of a box of 100 and they were all undersize by about .0005 (.2635). I too was having a hard time getting the performance I expected from the rifle so I emailed Berger. Their response was - "they are within the spec", nothing else. Ultimately I found 140 ELD-Ms and SMK 142 to shoot far better in that rifle.

 

[BoydAllen]

A friend routinely casts laps in his new barrels, before he starts machining them, to see what their groove diameters are. Over time he has run into barrels that varied from spec. more commonly on the small side (but occasionally larger) and when these were combined with bullets that were a couple of tenths fat, there has been an increased tendency toward picking up jacket fouling near the muzzle toward the end of long strings, which negativly affects accuracy . I do not believe that your FB diameter is the issue, because the very popular 6BRA's reamer drawing calls for a FB a full .001 over (.244) nominal bullet diameter. BT bullets do not swell up under firing pressure to the extent that FBs do. If you find that a good FB shoots well from your barrel, I would suspect that there is a mismatch between bullet diameter and groove diameter, which could be a couple of tenths over spec.

 

[Alex Wheeler]

Based on your post it sounds like you have only tried this one bullet? You need try try a few bullets and powders first. The fit in the fb is not the problem. If your groove diameter was on the large side and your shooting skinny bullets that can be. I have shot up to .244 on the groove playing with this in 6mm . Velocity and accuracy are both effected by the groove. Even at the bigger end it didnt shoot as bad as youd think, but it was not going to win any matches.

 

[gme]

are you measuring right on the "fire ring"? (where the bearing surface meets the transition to the boat tail) that's usually the largest spot, either way, if you haven't already i'd try experimenting with a few other bullets before giving up on the barrel

 

[Stumpy1]

Hi All,

Thanks for the responses so far.

Gary, I am measuring the shank -the ring is about 0.2642
Alex, I will try to slug the barrel to see what the groove diameter is.
WK -I will try some Sierras and see what happens

I will be trying other bullets and powders, but right now, as you know, stuff is hard to come by.

I will keep you posted as I learn more.

Thanks!

Frank