Bore diameter vs. bearing surface question

[reubenski]

So, I'm sitting around drinking beer and tinkering with a set of calipers. I noticed that the entire boat tail of a 7mm 180gr Berger Hybrid doesn't fit into Hornady bullet comparator I just used to measure my base to ogive OAL. I measured the inner diameter of the comparator insert. It is clearly marked 7-28. It actually measures at .274. A Berger 180gr Hybrid measures .2845. My rifle bore (Broughton barrel) at the crown measures .281.

Does all of this sound kosher?

Perhaps, I F'd up the rifle bore measurement, but the bullet measurement is a no-brainer as is the ID of the Hornady insert. Perhaps my caliper is off, but the Berger bullet measures correct. I'm sure the bullet compresses and is necessary to create pressure and velocity, but I never thought the bore diameter would be .281. That's 3 1/2 thousandths of bullet compression; doesn't seem unreasonable when you think about it. Still, why would the Hornady insert not measure .284 or .280 even? Seems like it is pretty far off the no kidding bearing surface.

 

[minnesotamulisha]

I don't measure bullets, in any fashion. However, I don't think it matters where you are measuring the bullet from, as long as you do it the same every time. My experience is limited here, but from everything I've learned and read, consistency is key. Hopefully others will chime in who do have experience with measuring bearing surface and bullet length.

 

[Bryan Litz Ballistics]

Sounds to me like when you measured the barrel diameter, one jaw was on the bore, the other on a groove. This would give you the result you observed. In fact, if you're barrel has an odd number of riflings (like 5), and you're measuring straight across, it would be difficult if not impossible to avoid doing it this way depending on the width of the riflings.

The bullet should be equal to the groove diameter (.284") at the bearing surface. Some bullets have a pressure ring near the base that can bump the diameter up 0.0005" or so. This dimension is intended to seal the barrel with very little compression or obturation (expansion). The bore diameter is smaller by 0.005" to 0.008" depending on caliber. In the case of 7mm (.284), the bore is 0.007" smaller than groove as DM stated above, which is 0.0035" per side.

Take care,
-Bryan

 

[spclark]

The bore diameter is smaller by 0.005" to 0.008" depending on caliber. In the case of 7mm (.284), the bore is 0.007" smaller than groove as DM stated above, which is 0.0035" per side.

So that leaves me wondering... is this groove depth something that's necessary for optimum bullet performance? Might there not be something to be gained by reducing bullet deformation by going to a larger bore diameter while keeping the groove diameter the same?

I have a friend who shoots the 155 Hybrid 308 Berger in 1:15 twist barrels at speeds above those common with 1:12 or lower. He does very well indeed with 'em too but says don't try this unless your barrel's got shallow lands, otherwise pressures are too high.

My thinking is that pressure is affected by how much swaging has to happen to make bullets fill the rifling. If shallower lands reduce the degree of deformation, without affecting bullet stability, higher velocities ought to be possible (without going up to unsafe pressures) with slower twists.

 

[mysticplayer]

Easy enough experiment... compare the bores of button and cut rifled barrels.

I bet you find button rifled bores to be slightly larger thus these would engrave less vs a cut rifled barrel. Groove diameters would be the same.

Get yourself pin gauges that vary 0.001" and just drop them down the bore. Very enlightening...

Jerry

 

[LHSmith]

So, I'm sitting around drinking beer and tinkering with a set of calipers.

Here's another experiment to try: Try chambering a barrel along with making custom bullets armed with only a set of calipers and report back how well that worked out for you.

 

[pdog]

A standard caliper will not measure inside diameter accurately. It is only designed to measure the distance between two flat surfaces. It is will only frustrate you. It is somewhat accurate when used to measure the outside diameter of a bullet.

You will need a micrometer and an inside micrometer to properly take these measurements.

pdog

 

[savagedasher]

Any of the barrel makers that has the Sunnen Honing system can make barrels that have .00005 Difference from front to back.. I got my first barrel that was straight. When it was chambered off the bore the front only had .007 run out. The same barrel maker sold another to my smith it was even straighter . Larry