9mm OAL Accuracy

[Steve D.]

Hello All,

Is cartidge OAL always going to be MOST accurate at .010 / .015 off the rifling for a Specific Camber in a progressive-press in production mode? I know people can tweak to .005 if they are checking each round but that's a bit much.

I ask because the OAL spec in my Hornady manual for a 124 HAP is 1.050 and an XTP is 1.060 but I'm sure my chamber will accomodate longer while still passing the "drop & twist" test. I see a lot of 1.115's etc. in posts here.

Gun is a Walther PPS (Great little gun and eats everything without protest, by the way). For a 2.4" barrel it is quite accurate to boot.

(An iteresting feature of this chamger is that it has a small Step of slightly smaller chamber-diameter just back from the "headspace-step" (or Stop) that keeps preasure in the barrel a little bit longer as the cartridge begins to disengage from full-battery. Brass stays Very Clean and you can clearly see the little "choke-ring" that comes a couple millimeters back from the mouth on the ejected casings...neat idea.)

Thanks for Any Input you would like to share.

[Jman]

The major concern with pistol ammo and OAL is safety. Trumping accuracy by a big margin.

>My< rule of thumb for handgun reloads is seat them a long as possible. A long as the magazine will reliably feed them.

FWIW...

[Hi-Power Jack]

Steve, my BHP is a little more accurate with a 1.12" OAL, but doesn't feed 100%

unless I get it out to 1.13". Lose a little accuracy, but gain reliability.

So, NO, it does NOT always mean that the longest OAL is the most accurate,

IMHO. And, as Jman said, depending on the gun and the load, longer might

be a little safer.

[Steve D.]

I agree in never going Shorter without serious consideration for the Safety of the load. I had to do this with my H-XTP's in both 185 gr. and 200 gr. for my H&K USP-45. Had to shorten them a good bit (I think 15-20 Thous. but can't remeber without my notes) to stay off the rifling for the "plunk-twist" from what was published in Hornady's manual. I now keep powder in the lower 1/3 of the range and the stuff shoots very accurately and comfortably.

It was nice to realize why my previous set of .45 loads was so harsh & nasty... They were all calipered to the Hrndy. COAL Book Spec's, but hard-up against the rifling! Woops...

Moral of Story is Always do the Plunk-Twist when setting-up the seating die for a new load, regardless of what the Manual says..

My question was Lengthening the 9mm Rounds to the .010 set-back from the rifling. The book value puts the length a fair bit shorter than I think it can be loaded to. If I'm at 1.060 and some guys here are at 1.115 or more that is like 50 +Thous'. That is quite a bit, no?

On the bigger question though, I think Hi-Power Jack probably has me covered on this so thanks again as always.

Steve

[mizer67]

Longer is not necessarily better in pistol ammo.

[Red Ryder]

Longer is not necessarily better in pistol ammo.

I have had the same experience with my Glock 17. It just plain shoots more accurately at 1.135ish compaired to 1.150ish with 124grain MG CMJs and N320 using a ransom rest. repeatable results. Powder and power factor affects accuracy also. My accuracy with Titegroup sucked at 130 PF but improved significanly at 140PF, regardless of AOL. good luck. test and retest, then verify by testing again. I chronoed with a ransom rest someone had on chrono day at the range.

Edited December 15, 2012 by Red Ryder

[rhyrlik]

You can use OAL to fine tune your loads. But just because the bullet is close to the rifling does not necessarily mean that OAL will shoot the smallest groups. My fav OAL with the 124 XTP is 1.080", but that's with a certain powder at a certain charge weight.

[Beacon22]

Will a longer OAL for a 147 JHP in a 9mm case lower the pressure because there is more case volume?

[Justsomeguy]

Will a longer OAL for a 147 JHP in a 9mm case lower the pressure because there is more case volume?

Here's the thing... Yes and no. Yes, it can lower the pressure slightly when you are at full power loads EXCEPT if you are so close to the rifling that the bullet cannot move before the powder is completly lit up... then pressure will spike. So, if you are thinking you have a pressure problem change powders to a slower burner than you have and adjust the OAL for accuracy. Stay off the rifling by at least .005 or more. Rifle loadings can be right into the rifling and shoot well because the powders are so slow. All pistol powders are much quicker and you have to stay off the rifling by a good bit.

[Beacon22]

Will a longer OAL for a 147 JHP in a 9mm case lower the pressure because there is more case volume?

Here's the thing... Yes and no. Yes, it can lower the pressure slightly when you are at full power loads EXCEPT if you are so close to the rifling that the bullet cannot move before the powder is completly lit up... then pressure will spike. So, if you are thinking you have a pressure problem change powders to a slower burner than you have and adjust the OAL for accuracy. Stay off the rifling by at least .005 or more. Rifle loadings can be right into the rifling and shoot well because the powders are so slow. All pistol powders are much quicker and you have to stay off the rifling by a good bit.

That makes sense. I was wondering why my spent primers were filling the primer pocket to capacity and super flat looking. Someone I shoot with had picked up some of my brass and looked at the spent casing and said that I was running too high of a pressure based on how the primer looked.

I load a 9mm 147hp zero over 3.2 of universal clays to with a maximum OAL of 1.155. I guess I am getting that pressure spike your talking about.