"crimp" And Power Factor

[jgobbels]

I chronographed 3 strings of 10 rounds each tonight and found some strange results relating to PF with different "crimp" measurements.

My 1st string of 10 rounds had no measurable taper. 9 out of 10 rounds made Major~

My 2nd String of 10 rounds had .003 of additional taper. I had only 6 out of 10 round make major~

My 3rd String of 10 rounds had .005 of additional taper. I had only 6 out of 10 rounds make major~

I would have guessed that an increase in taper would have increased my pressures, thus increasing FPS. The above seems to work against that idea.

Loads were:

.38 Super

115g Hornady XTP, JHP

Federal Small Rifle Primer

8.5g of VV-N350

OAL 1.217"

This was my first set of loads for super so I'm a novice at this. Hopefully someone can help me out here to understand what is going on.

Also, what is a good fudge factor relating to PF. My average on my 1st 10 rounds was 165.876. Should I shoot for a minimum of 169-170???

Thanks in advance,

John~

[Loves2Shoot]

If it takes a lot of energy to make the bullet leave the case (ie. tight crimp) then you don't have that energy to push the bullet out the barrel with. I might be wrong on that, there are enough engineer types on hee to give you the scientific explanation, but that would be my guess.

[EricW]

You also may be effectively reducing the bullet's friction with the additional crimp simply because you're squishing it down on the OD. A taper crimp isn't a crimp anyway. It does nothing to keep the bullet from setting back. Only crimp enough to make the case smooth and feed properly.

[TDean]

I don't think that experiment means much j.

Statistically, you shot 30rds and 9 of them didn't make "major" that's all.

I don't think crimp had much to do with it.

It seems you're splitting hairs.

For the "fastest" group of 10 rds, your average squeeked by at 165.876 PF

What are you calling "minor", 163?

Ya, aim for 171 and forget about a +/- 5.0 PF

[jgobbels]

Thanks for all the responses. I'll keep my crimp at .378 and get my power factor to 171. Thanks for all your help guys.

Its just confusing having some tell me to crinp and some telling me to resize to factory specs. I just wanted to hear it from the experts.

Thanks~

[vluc]

I've found that keeping crimp constant and changing oal affects it more. According to the folks at Hodgdon - taken from an e-mail to me from Phil

The powder charge will be the cause of the most change.

Thanks,

Phil

[joshua]

8.5 grains of VV N350 and it's only breaking 165 PF. You must have many holes on your barrel and must be the commander length type. I make old major (with lots of cushion that I know I'll make major anywhere in the US) with just 8 grains of N350. Have you checked your powder measure to see if it's constant with it's throw? josh

[jgobbels]

Joshua,

I checked every 5 rounds on the scale and they were all 8.5 to 8.4grain. I guess the scale could be incorrect Lyman-1000 so I'll dig up my old beam scale to double check.

The gun has a 3 port hybrid and a 6 port comp. I guess a bit too many~

Thanks for the response,

John~

[joshua]

It's probably the hybrid ports requiring the extra grains of powder. I forgot about how much porting goes on barrels nowadays. Josh

[jgobbels]

Well I'm not 100% sure.

The gent I purchasd the gun from stated that 8.5gr of 350 behind a 115 JHP made major????

I guess it does in some sense, but not the 170-171 that I need. Could the OAL be something that could cause a decrease in my PF?

I'm using the Hornady XTP JHP 115gr with an OAL of 1.215.

Any help would be appreciated.

Regards,

john~

[Chriss Grube]

OAL has a lot to do with it. Shorter higher pressure more FPS, Longer less pressure slower. The bullets can make a difference also. Hornady encapsulated barely made major same load with a regular bullet 170. Some seal the bore better than others. Call the guy up and get his load exactly right down to lot numbers.