What's a standard jump for pistol bullets?

[GJgo]

Where do you start your jump length when working up a load for a new pistol? For rifles with lead core bullets I always start at .015" off the lands & tune from there. Same? Different?

I'm starting a new load using 124gr XTP in a 9mm, if it makes a difference. Where it hits the lands in my pistol is .055" longer than the book COL, which seems excessive for a custom handload.

 

[Otto]

Depends on the gun and the bullet.
For my CZ 75B, with Zero 115gr, I have to load to 1.100 oal or less. Anything longer, say 1.105 and the bullet gets stuck in the lands of the barrel.
My Sig 226 and 228 don't have this issue.

 

[GJgo]

Thanks. Note though that I'm not talking about COL, since the lands aren't in the same place on every barrel. I'm looking for a general jump you'd start at (to determine COL) once you measure where the lands are in your barrel.

 

[rfwobbly]

I start at .010" PLUS an amount that reflects the OAL variations I've measured in my progressive press.

The "book COL" could include many trade-offs, like chambering in numerous pistols. To underscore that point I can tell you that the "book COL" is way too long for my CZ. Also you'll want a goodly percentage of the bullet seated behind the crimp, and as always the round has to measure under 1.169" to get into most mags.

If you are OK to extend the COL, then at that added length (.055 -.010 -press variation) you may find you need ever so slightly more powder to achieve their listed speed.

Should be fun!

 

[GJgo]

Thank you.

In my M&P 9c, the 124 XTP hits the lands at 1.115" OAL. I think I'll start at 1.100" and work on some charge weights.

 

[1SOW]

rfwobbly:

Also you'll want a goodly percentage of the bullet seated behind the crimp,

I''ve tried to find a desired seating depth for 124gr jacketed 9mm (using 4.2 n320) and haven't found anything specific. Mine end up seated about .20X - .22X depending on the bullet.

Does this sound in the ballpark? After feed and function tests, How do you know it is or isn't enough/too much?

 

[rfwobbly]

In my M&P 9c, the 124 XTP hits the lands at 1.115" OAL. I think I'll start at 1.100" and work on some charge weights.

That sounds good for a XTP since the nose is so "clean" in its design and manufacturing. I might balk at that number with a bullet like Montana Gold since a lot of their bullets show a puddle of lead on the nose that seems to vary in height from bullet to bullet.

Remember: You are measuring the OAL, but it's the shoulder of the bullet that's touching the rifling! So by pushing on the tip you are not directly controlling the shoulder location during seating, nor are you directly measuring the shoulder location on the finished product! Without a really good bullet like the XTP you're not going to be able to predict where the shoulder might be.

But that's just my 2 cents.

 

[rfwobbly]

I''ve tried to find a desired seating depth for 124gr jacketed 9mm (using 4.2 n320) and haven't found anything specific. Mine end up seated about .20X - .22X depending on the bullet.

I've never seen a number either, but right off I'd assume 3 things....
• When figuring the percentage of bullet inside the case, we probably want to focus on the portion of the bullet which has the full diameter, and ignore the tapered portion of the bullet's nose
• Of that portion I'd say you want at least 50% inside the case, and hopefully far more
• Since the main reason for this support is to keep the bullet from getting knocked off-center when it hits the feed ramp, then the longer the reduced diameter portion is, the more you want to increase the amount of bullet inside the case.

So when you have a long slender nose like this (acting as a lever)....

...then the further back in the case you'll want to seat.

Again, just my 2 cents.

 

[Walkalong]

Most people just seat to a length that feeds well and shoot em. Unless you have a very accurate gun, and can shoot extremely well (which some do and can), I doubt you will be able to tell the difference in accuracy from any reasonable seating depths.

 

[rfwobbly]

It's a moot point for 1SOW and myself since we shoot CZ pistols. The freebore is so short on a CZ that the primary concern of getting the slide into battery seats the bullet WAY back into the case as "Step #1" whether we like it or not!

 

[ranger335v]

"What's a standard jump for pistol bullets? "

There is no such "standard" for either pistols or rifles.

 

[243winxb]

In autos,start with the maximum COL for the caliber so it feeds/fits the magazine, adjust accordingly. For rifles, have the base of the bullet, full diameter bearing surface, at the neck shoulder junction for starters. Jump means nothing to accuracy. Centering the round in the chamber does. With lead pistol bullets, seating just into the leade of 45acp types, will take end play out of the loading, giving better accuracy.

 

[GJgo]

Remember: You are measuring the OAL, but it's the shoulder of the bullet that's touching the rifling!

Correct. I'm seeing about a .015" variance in OAL of the XTPs themselves, so I'll have to keep that in mind.

 

[ants]

Seating the bullet close to the lands

The benefit in a finely tuned target rifle with finely tuned ammunition is generally quite positive. The benefit in a beat up old range rifle is negligible. Your experience may vary.

The benefit in my handguns is non-existent. OAL is entirely a matter of keeping the bullet shoulder off the lands, fitting the magazine, and feeding reliably. Your experience may vary.

Bottleneck rifles run at much higher mean average pressure, and headspace on the shoulder of a case with a carefully trimmed mouth to achieve consistent trim-to case length. Internal ballistics are very, very different from pistol/revolver ammunition. The characteristics that gain a benefit in target rifles, is generally not found in most pistols. Your experience may vary.

Don't forget that your straight-wall auto pistol ammo headspaces on the case mouth. As your case length varies, the distance between bullet and lands will vary. Unless you trim every case, this generally negates very careful attempts to use OAL to place the bullet very near the lands.

 

[Walkalong]

Agreed. See post #9.

I would like to see the shooter could could prove the differences without a machine rest.

 

[ranger335v]

"The benefit in a finely tuned target rifle with finely tuned ammunition is generally quite positive."

My experience does vary and not in "beat up old range rifles", whatever that is!

The proven benefits of loading close to the lands in single shot target rifles (BR) is variable but they frequently do gain some from that practice. BUT, the "carefully tuned" ammo for those tightly chambered rifles is NOT the same as we typically use in factory sporters.

Seating in the lands is not magic. BR bullets are typically lightly held in thin turned necks. Seating close or even hard into the lands with those rifles helps the load to attain proper ignition and peak pressures to assure a proper powder burn with the best powders for the cartridge. (It's really NOT to help the bullets to enter the rifleing straight as convenitonal wisdom suggests, but it MAY help that, a little. Fact is, concentric ammo will feed bullets into the lands quite consistantlly no matter the jump.) Thus, Walkalong's observation in his #9 post applies.

Normally large, to the point of being sloppy compared to BR rifles, our factory chambers allow standard - thick - factory case necks to hold onto bullets much tighter. Thus factory sporters DON'T benefit much, if any, and that rarely, from setting bullets against the lands because doing so can raise peak pressures more than is needed for the rest of the powder burn, potentially degrading accuracy.

Bottleneck cases run higher pressures because they're rifle cartridges, not because of the necks. Both rifles and handguns benefit from having a proper time-pressure curve but that's normally achieved by choosing the proper burn rate powder and primer for the load, not how they headspace.

YMMV.

 

[GJgo]

Don't forget that your straight-wall auto pistol ammo headspaces on the case mouth. As your case length varies, the distance between bullet and lands will vary. Unless you trim every case, this generally negates very careful attempts to use OAL to place the bullet very near the lands.

That's a good point. maybe then, I should just set it to the OAL in the manual and run with it?

 

[ranger335v]

"That's a good point. maybe then, I should just set it to the OAL in the manual and run with it?"

Try it. If it functions and chambers, run with it. If not, change it until it does.

 

[1SOW]

rf: Your conclusion about seating depth makes sense.

It's a moot point for 1SOW and myself since we shoot CZ pistols. The freebore is so short on a CZ that the primary concern of getting the slide into battery seats the bullet WAY back into the case as "Step #1" whether we like it or not!

I tried the chamber max lngth test on some PD 124gr fmj I was giving my son for his Shadow. It measured 1.224" to juuust touch the rifling. That leaves a lot of variation in oal/seating depth choices available after reducing to a feedable[] size..

ants:

Don't forget that your straight-wall auto pistol ammo headspaces on the case mouth. As your case length varies, the distance between bullet and lands will vary. Unless you trim every case, this generally negates very careful attempts to use OAL to place the bullet very near the lands.

You're right of course. Their are a lot of variables (bullet length and ogive variances for one) that either can't be controlled or that aren't worth the effort. By limiting OAL variations it just reduces one of the possible variables.
I read a forum comment by Angus Hobdell where he stated he normally used 'Zero' bullets for competition; but at International or BIG matches, he used Speer because they were slightly more accurate. He believes he was limiting one of the variables. How much difference would that make at 7-15 yds?
I bet it's not measurable.
IMO, I'll also bet that "confidence" in your round (OAL tweaker) can make a BIG difference in how you shoot.