Crimp math?


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layusn1
August 3, 2007, 04:16 PM
I have seen the numbers before but I don't recall where...isn't there an equation to figure out how much crimp you should put on a round i.e. how much a round should measure at the diameter of the neck after you apply the crimp? I think it had to do with the width of the brass at the neck but I don't recall the exact details. I am getting ready to set up my Lee FCD for 243 Win rounds going in a bolt rifle so I want to make sure I have everything as "perfect" as possible. If anyone knows I appreciate your sharing.

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cdrt
August 3, 2007, 04:38 PM
I have some factory Winchester's that measure .265 across the case mouth. My reloads measure .268. I think max is .276.

You really don't need to "crimp" them...a pressure fit should be enough.

I use the rule of thumb to add .02 to the bullet diameter to get the right "crimp". e.g. with .452 bullets in my .45 ACP, I taper crimp to .472 or .470.

Make sense?

USSR
August 3, 2007, 07:06 PM
isn't there an equation to figure out how much crimp you should put on a round i.e. how much a round should measure at the diameter of the neck after you apply the crimp? I think it had to do with the width of the brass at the neck but I don't recall the exact details. I am getting ready to set up my Lee FCD for 243 Win rounds going in a bolt rifle so I want to make sure I have everything as "perfect" as possible.

layusn1,

You don't want to crimp rounds for a bolt rifle. The only thing you can possibly accomplish by doing this, is to distort your bullet with a negative impact on accuracy. Unless you are using a rifle with a tubular magazine, secure the bullet by creating the proper amount of neck tension.

Don

Bad Flynch
August 3, 2007, 07:14 PM
The maximum and minimum cartridge dimensions are all specified in the SAAMI drawings and are not a calculated function, except that the dimension should not exceed maximum specs.

Crimping is fine on the right combination; crimping is useless or worse on an incorrect combination.

layusn1
August 3, 2007, 07:15 PM
So I bought an FCD for nothing? Do I just do the push test against something hard to see if the bullets stay put? Or chamber them and measure again to make sure they didn't change OAL?

Walkalong
August 3, 2007, 07:23 PM
So I bought an FCD for nothing? I would say so, but many love them.

A crimp on a bolt rifle is not really necessary if you have sufficient neck tension.

I have never measured a crimp. I eyeball them. My .45 ACP crimp is so light I have to use an eyepiece to see it. A .44 mag crimp on a slow burning powder will be very heavy and roll the case deep into the cannelure or crimp groove. Once I get a crimp die setup and the load shoots well I leave it alone. I trim my revolver brass so I get a good consistent crimp and if I change brass. I have the trim length recorded so I can duplicate that crimp with the new brass.

USSR
August 3, 2007, 07:33 PM
Do I just do the push test against something hard to see if the bullets stay put? Or chamber them and measure again to make sure they didn't change OAL?

The push test will work. You don't have to use alot of pressure either. The .243 is not exactly a hard recoiling round, so the bullets of the rounds in your magazine moving is not likely unless you can move your bullets around with your fingers.

Don

LHB1
August 3, 2007, 08:51 PM
Layusn1,
Perhaps the 'formula" you were thinking of is:
Diameter of loaded round at neck should be 2 times the thickness of brass in case neck plus the diameter of bullet.

If your loaded rounds measure more than that, you have some airspace between the case neck and bullet. If your loaded rounds measure less than that, you have somehow managed to compress/deform the bullet.

Good shooting and be safe.
LB

layusn1
August 3, 2007, 10:08 PM
Layusn1,
Perhaps the 'formula" you were thinking of is:
Diameter of loaded round at neck should be 2 times the thickness of brass in case neck plus the diameter of bullet.

If your loaded rounds measure more than that, you have some airspace between the case neck and bullet. If your loaded rounds measure less than that, you have somehow managed to compress/deform the bullet.

Good shooting and be safe.
LB

That might well have been it and I misunderstood its purpose. It sounds familiar. Thanks.

jhansman
August 3, 2007, 10:32 PM
So I bought an FCD for nothing?

Perhaps not. Many bolt shooters still crimp, claiming it aids in accuracy. Heck, you have the die, why not try it out? I don't crimp my .223, but so many rave about the Lee factory crimp die, I sometimes wonder if I'm missing out.

Sunray
August 3, 2007, 10:52 PM
You don't need to crimp a .243. Nor do you need to crimp most other cartridges. Neck tension is enough.
Crimping is really only necessary for heavy recoiling rifle cartridges and cartridges used in lever actions. A taper crimp on the .45 ACP aids feeding, but otherwise don't bother.
However, if you opt to crimp anyway, use just enough to hold the bullet in place. Less is better as crimping is detrimental to accuracy. There's no magic formula. It's strictly trial and error.

GunTech
August 3, 2007, 11:52 PM
Do you mean crimp, or neck tension? I don;t crimp excpet for the heavies (458 win mag, etc). For my rifles I like 0.001-0.002 neck tension.

layusn1
August 4, 2007, 12:28 AM
Just out of curiosity...would there be any value in loading my developmental rounds in a non-crimped and slightly crimped version for the same powder charge? It's starting to look like an overwhelming vote against crimping period and I would hate to waste 40-ish grains of Varget per round just to find out what experience has already shown. I'm already going to go into shock over how quickly the powder will disappear. The addition of 223 after 9mm was a huge shock and it looks like 243 Win is about 15-ish grains more than 223. I would hate to see how quickly a pound of powder goes if your loading 50BMG...Now I understand buying 8lb jugs. I never appreciated that when loading 9mm. Time to start saving for the big jug...lol.

jhansman
August 4, 2007, 01:43 AM
would there be any value in loading my developmental rounds in a non-crimped and slightly crimped version for the same powder charge?

Why not make up dozen or so rounds of each and do a double blind study yourself. Go to the range and have someone else load for you. Compare the results and you decide. Yeah, it'll cost you some powder, but then you'll know, won't you? Personally, I think the results would be most interesting.

layusn1
August 4, 2007, 02:48 AM
That might be worth a try. It might be better to settle on the best load for my rifle though and then compare that load to rounds with various amounts of crimp...that would save some powder instead of doubling up every load.

Walkalong
August 4, 2007, 09:20 AM
Just out of curiosity...would there be any value in loading my developmental rounds in a non-crimped and slightly crimped version for the same powder charge?

Yes.
I lighly taper crimp my .223 loads for plinking. H335 Winchester 55 Gr. FMJ. I like to crimp .223 rounds since I am shooting them in an auto loader. Many folks do not crimp their .223's with great success. If I was shooting for pure accuracy I would test them thoroughly both ways and shoot whichever did best.

CZ57
August 4, 2007, 11:09 AM
layusn1: for handgun loads, maybe this is what you're asking about.
Casewall thickness X 2 + bullet diameter - amount of crimp desired.

Here's an example that is common for me using .451" jacketed bullets with the casewalls measuring .011".

(.011" X 2 = .022") .022" + .451" - .002" (the crimp I use for most .451" JHPs) = .471" diameter after taper crimp.

I get cases that often measure .0105, so adjusting, diameter after taper crimp would be .470".

For .40 S&W, I usually apply .002" of taper crimp also. But, with either .40 or .45, cast lead bullets will be at least .001" oversized so I only crimp .001" or crimp just enough to remove flare and nothing more.

For 9mm, .0015" is the maximum I use. The reason I use these numbers because this is the amount of taper crimp that will prevent setback when doing a bench/push test, but I am a bit heavy handed!;)

peterotte
August 4, 2007, 10:07 PM
:uhoh:Umm.... here comes the 'spanner in the works'. I don't even re-size!:what:

:scrutiny:Can I come out now?

I use a paper hand towel sabot to hold the bullet in the neck. This holds pretty tight in the 303Brit and only just tight enough in the hornet. I notice no difference when firing the 303Brit but the hornet changes it's point of impact but not group size. Pressures do seem higher in the hornet - possibly to do with bore sealing. And the paper disintegrates completely. I have not chronographed my loads yet. (My method is too much trouble for bulk loading and not necessary for long life brass like 223).:)

So the point is consistant grip on the bullet.

Peter

Stinger
August 4, 2007, 10:22 PM
peterotte,

That is some interesting information. I'm not interested in doing it, but I'm always interesting in learing new and unique information. I've heard of benchresters not resizing, and simply seating the bullet and using the bore to hold the tension on the cartridge. But I have never heard of anything like what you mentioned.

peterotte
August 5, 2007, 12:16 AM
Stinger

Thanks for that information. That is something I wondered about.

Regards
Peter

45ACPUSER
August 5, 2007, 12:46 AM
There is absolutely NO need to crimp bolt action rifle rounds! NOTTA! except perhaps heavy recoiling rounds.

layusn1
August 5, 2007, 01:40 PM
I wish I had gotten to test those rounds this morning... I loaded up 50rds of 243 Win last night and I was really looking forward to sending them down range. Took the AR15 and the 9 for good measure. I made the 40 minute drive out to the range and spent 2 1/2 solid hours picking up brass...not scrounging...it is all lying there out in the open. I got myself a 5 gal bucket of brass. The 10mm fairy even dumped some Norma brass on the range for the first time ever and more 223 than I have ever seen at the range. The only problem is it is an outdoor range and I underestimated the San Antonio heat this morning. By the time I was done I was fit to be diagnosed with heat stroke and never got to fire a single round... Oh well, on the bright side, I've got about 70lbs of brass to go through. Maybe I will get to try that experiment in another couple of weeks.

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