# Bullet Weight vs. Bearing Surface

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by padd54, Dec 18, 2010.

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Which is the more critical measurement, weight or bearing surface when segregating bullets?

I have separated bullets by .001" bearing surface, then weighed. I am finding a variance of +/- .2gr. within each group.

This is using the SMK 300gr. bullets.

Thanks,
Ray

Last edited: Dec 18, 2010
2. ### MikecrWell-Known Member

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You could analyze every possibility here, and gain nothing with it (other than understanding).
A 300smk is both really heavy, and really long. So this variance amounts to microscopic percentages(0.2/300 = 0.07%).
Then there is the possibility that you're not actually isolating bearing, but getting some base angle and/or ogive radius variance affecting your datums of measurement. So then the question is which specifically, by exactly how much, and what are the affects of every combination for real.

I wouldn't worry about it in this case.

Last edited: Dec 18, 2010

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Thanks, but I want to assemble the "perfect" cartridge.

So, with that in mind, which measurement would be the more critical and have the greatest impact on ballistics?

4. ### royinidahoWell-Known Member

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The goal is to achieve an Extreme Spread in very low teens or single digits. The lower the better.

Bearing surface variations is commonly suspected as the culprit. Vertical stringing at long distances is proportional to velocity spread.

However, variations in bc are also a factor. Thus included with bearing surface measurement is meplat trimming. There are those here that say it makes a difference beyond 600 yards. I tend to agree with them.

A bit poorer but uniform bc is certainly better than a varying bc from shot to shot.

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Thanks, so I need to trim the meplats(tips) after I determine their Bearing Surface and disregard the weights ?

How much do I take off, and do they all need to be the same OAL?

I don't mind doing the detail work, it will be awhile before the weather is mild enough for me go out shooting.

6. ### MikecrWell-Known Member

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Well, this side of carefully known extremes, AND competent ballistic lab validation, it's all conjecture and folklore.
It could not even be guessed at without painstaking isolation of each specific variance.
For example, a bullet that is heavier 'might' have a lower MV, but a higher BC, combined with a barrel time that releases a bullet on a higher trajectory. Or it may have a higher MV depending on the grove/bore seal and friction, and bullet's extra weight contribution to it's bearing or diameter or length, and/or resulting case fill/powder. This of course completely changes if your bullet's are coated, and might help if the extra .2gr is producing a smaller meplat..
More bearing might mean anything to YOUR bore/groove. It might like the sealing, or hate the friction. The small tension change from extra bearing seated, could put you in & out of an edgy tune.

Every combination of internal and external ballistic affects here could sum to zero, or an extreme, and it's unpredictable without isolation and local validation.
You're soliciting conjecture.
There, I said it...

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Thanks, now I completely understand and can sleep easy.

And you are correct in your conclusion, I am soliciting conjecture. That is the purpose of 90% of the posts on any forum I would say.

8. ### royinidahoWell-Known Member

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Those that are paying attention to this detail AND winning matches are doing the above.

There is a reasonable point beyond which no noticiable improvement will be noticed. I suspect that the reasonable point progresses with distance being shot.

I'll be doing this process soon. These will be my steps:

1: Trim all bullets to the same length. (Base to meplate). Starting with the nastiest looking nose and some of the SMK points are really lopsided.

I don't think I want a meplat larger than .05, which to me is too large.

2: Not sure on this step until I try it. It will either be sort by weight or bearing surface length. The small amount of this sorting shows that overall bullet weight varies with bearing length and vise versa.

If I sort first by bearing surface length I'll then sort these by weight.

If I sort first by weight I'll sort these by bearing length.

I'll do this until it proves to me that it doesn't make any difference then I'll bag the whole idea and just go kill stuff.

9. ### GeneWell-Known Member

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I use mostly Bergers and have found that the slight differences in bullet weight are not enough to be of concern, so I no longer weight them. Bearing surface is another matter. It affects the time the bullet is in the barrel, and is more critical. I separate those bullets and keep any with a .003" and less variance in BS. The others are my sighters, practice bullets, etc.

I shoot 500 meter (approx. 550 yds) groundhog matches. I trim meplats for 6mm, 6.5mm and 30 cal. bullets. I have not seen any great difference at extreme ranges, but it may be an advantage to do so at longer distances. The meplats need just a slight touch to uniform them, reducing the length about a thou or two. But then, it is important to open the point slightly with a tiny inside reamer. Meplat pointing will cause a small reduction in BC. It is not necessary to trim meplats so that all will finish with exact uniform total bullet lengths.

The newest fad is bullet pointing, which I have not tried. Seems like every month or so a new "super gadget" comes along, and I am getting weary of being the sucker for all of them.

10. ### royinidahoWell-Known Member

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You too, eh!

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Where would I get a "tiny reamer" to open the hole back up?

If I had one of the bullet pointers, would I still need to trim the meplats?

Thanks for the education,
Ray

12. ### GeneWell-Known Member

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I have Kevin Cram's (Montour Co. Rifles) meplat trimmers, but others are similar. As stated above, I found no value in meplat trimming - other shooters disagree. You place a bullet in a hollow steel chamber of proper diameter, and the cap has a fluted cutter which you set to a fixed position; then twist cutter against the meplat. This reduces the meplat a few thou, hopefully so that all bullets measure the same total length. But in so doing, it flattens the hollow point. The point must be reopened, using the same chamber tool with a tiny reamer you twist and open the hollow point so that all are uniform. Here is an article and photos regarding meplat trimming.

Review of Montour Meplat Trimmer

A pointing tool, which I do not use, threads into your press similar to a sizing die. Squeezing the bullet into the pointer compresses the meplat, making it more aerodynamic. But you will need to uniform the meplats first. Here is a good article on the pointer:

Whidden Bullet Pointing Die within AccurateShooter.com

I suggest you read both articles carefully and note that results with these methods is very controversial.

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