Bearing surface measurement

[johara1]

Why is bearing surface measurement taken from the ogive to surface of the boat tail with a comparator verses ogive to the base?........jim

 

[Laurie]

Jim,

so far as I can see, the rear end is usually kept pretty consistent. It's the front end that sees the variations, so you want to batch them or cull out the odd out of spec examples on that basis. I'd imagine the practice started with flat-base BR type bullets anyway, and you can only measure base to ogive here.

If bothered, you can buy two Hornady or Sinclair comparator bodies and calibre bushings attaching one to each calliper jaw. Even that doesn't solve the problem entirely as comparator bushings have smaller apertures than the full bullet bearing surface diameter. The one fitting the rear end of the bullet will stop before it reaches the bearing surface and is still on the boattail section, but there is less scope for error presumably. The logical answer here is to ream out the bushing until it is marginally short of the full bullet diameter so you'd measure from body-boattail junction to a point on the ogive.

I'd say that doing this would be getting seriously nerdy though, unless anybody can prove otherwise. A quick check and batch is all that's needed, and many very successful long-range shooters say don't bother with that even, as you don't see any results on the range. 1,000yd Benchrest might just about justify it I suppose.

Laurie,
York, England

 

[therifler]

the body dia . has been known to vary in lenght with some bullet manufactures . which will increase or decrease your pressure on varing lenghts and cause a change in point of impact . or so i have been told i don't measure my own . T.R.

 

[johara1]

Ok guys here it goes i use tubb bearing comparator to measure the bullets and i was told this measurement is misleading because of how the comparator contacts the boat tail,and that a ogive to the base is more valid measurement. so what do you think? A bearing surface thats .020-.030 doesn't matter as long as the ogive to the base is the same......jim

 

[johara1]

This is what been happening to me, 115's out of a dasher i started with a square mark on the bullet and 33 gr of rl 15 it shot good. Once in a while a flyer not bad,but it would open it to .3-.4,so i started to check the bearing surface to see how consistant they were. I found that the main size i set for zero the next plus .005 and the last plus .020 all of these are +or- .001.
So i email berger about this and was told that isn't the way too measure them,ogive to base is the correct way. I guess i'm just stupid and bearing surface length doesn't mean a rats rear end..........jim

 

[fullersson]

I'll have to disagree with what you were told. What one is trying to measure with a bearing surface comparator is.... bearing surface. That part of the bullet that comes in contact with the rifling.

The boat tail doesn't contact anything so, as Laurie pointed out, what you would really want is a land diameter opening in both parts of the Tubb tool. While it doesn't do exactly that, it's still helpful. Berger bullets are normally very consistent within a given lot, but from lot to lot the measured bearing surface can vary significantly. I recently bought a large quantity of their 105 BT's that measured 0.075" longer in bearing surface than the earlier lot (not a typo, seventy-five thousandths longer). With a 0.2 grain reduction in powder I still have pressure signs that didn't happen with the shorter ones.

As far as sorting Bergers by bearing surface from the same lot I decided it was a waste of time. Sierras are a very different story however.

Gunamonth hit the nail on the head.

 

[johara1]

Gunamonth,
The bullets that i have are from the same lot and they vary over .020 and an other lot of 105's vary that much
also.........jim

 

[JERRYHM]

I have several store bought tools to measure Bearing Surface ---- They all measure to different points on the bullets and are measuring at the wrong place. By turning the bullets in the tool you can see where each tool measures and all of them were different. VLD bullets are more critical in their measurement location.
I NOW use a short (about 1 inch long) barrel stub chambered with my reamer just enough to be able to place a loaded round in the stub without the case touching the stub. The stub allows me to measure both bullets and loaded rounds. The loaded rounds can be adjusted very accurate in their relationship to your lands.

I'm more interested in the bullets ogive to land measurement.

I think Berger's bearing surface tolerance is < 0.003" per lot.
Using the barrel stub I have not found any over 0.002"

 

[RBD]

Sadly, Jerry, I'm not certain everyone on the forum knows who you are, but... it's important for them to know that if you read and believe, and follow the advice of anyone on this forum, it should be Jerry.

We discussed this at Sacramento last year, and I'd like to get it right for myself and the rest of the readers.

If you have your 'smith cut a chamber in a barrel stub--this should be a stub from the same barrel as your rifle, so the lands are uniform--you should then have him cut the stub off between the point where the ogive of your bullets hits the lands, and the point where the forward most part of the case would sit in the chamber.

Using this stub, one would then sort the bullets for ogive to base.

And, had one a micrometer seating die, he could use the same stub to ensure--by measuring the cartridge in the stub--that all rounds were seated with the bullet ogive the same distance from casehead (or bolt face) to ogive.

Thanks, Jerry-

My only question--and I know you don't suffer fools gladly--is why, when measuring for case OAL, it's important to use a stub cut short, so the case doesn't actually touch the false chamber?

Best,

RBD

Is my problem that I don't really understand the term "ogive," and have always thought that this is that part of the bullet that touches the lands?

 

[tenring]

Bought a Hornady comparator and did not like it. Then I ordered a Davidson Seating Depth checker from Sinclair. Its easy to measure bearing surface by using two nose pieces of the same caliber on your caliper. Line them up and go for it. Place the measured bullets in plastic cups with the bearing length marked on a slip of paper.

While you are at it, buy the Davidson base. With the base and appropriate nose piece, you can measure OAL to the ogive on a seated bullet.

 

[johara1]

Jerry,
I use David Tubbs BSC. to measure and i also use to measure ogive to base on the 108's they all +-.001 on the bearing surface measurement and the same +-.001 on the ogive to base measurement. I also use a stub for the over all length of a loaded round and to check shoulder set back.

The 115's and 105's are an other story,it's like they threw three different lots together........jim

 

[eitmsmf]

Check this tool out. I've been using it for awhile now and am very happy with the results. Its on the new products page.

www.greensrifles.com