Determining Max OAL

[Laughingdog]

Having purchased a CZ-75 SP-01 earlier this year, I was forced into a crash course on determining the max OAL for various bullet profiles. The Lone Wolf Barrel in my previous primary gun was reamed so aggressively from the factory that just about every bullet profile out there could be loaded to 1.169" and still have a very large gap between the bullet and the start of the lands. But the CZ, by comparison, is ridiculously tight.

Note: I couldn't figure out how to embed photos of this from my Flickr account, so I just embeded links to them in here.

The whole process of "make a dummy bullet, drop testing it, seating it lower, drop test again, etc." for one bullet type after another got really tedious, and the guides I saw suggesting making cuts on a brass, putting in a bullet in, and pushing it into the barrel seemed excessive, especially with those same guides stating that the bullet might just get pulled back out part way by the rifling when you remove the case. So, being both an engineer and a bit lazy, I spent some time during a particularly boring meeting at work coming up with something that was both easy and consistent.

It occurred to me that if you take the length of the bullet, drop it in the barrel and measure how deep it sits, and then add on how much of the case hangs out of the barrel, you have the maximum OAL possible for that bullet in that barrel. Take off 0.015"-0.020" from that, and you have something that's short enough that some tolerance stacking from variations in case length and slop in your press won't screw you.

So, to start, you'll need your barrel, a micrometer, a piece of sized brass, and one of your bullets.

Step 1: How much brass hangs out of the barrel when the case is resting on the lip in the barrel

There are two ways to do this. The first is easier, but I've found it to be harder to get consistent measurements. Regardless of which method you go with, the first thing to do is to measure 5-10 pieces of brass to see how much they differ, and then base your measurements on something at, or a little below, average. The reason for this is that the brass head-spaces off the end of the case. So for a given OAL, the shorter the brass, the more of the bullet hanging out the end, and the closer the bullet will be to the rifling.

The first method is to just drop the case in the barrel, and then measure how far it sticks out past the side of the barrel.

If your gun is like my CZ, the left and right sides are different heights. So make sure you're consistent.

The second method, and the one I prefer, is to measure the depth of the lip the case would rest on, and then just subtract that from the length of the case. 9mm cases ranged from 0.743" to 0.75" for me, so I used .0745 for my numbers. The lip on my CZ barrel was 0.635" down, so that gave me a delta of .011".

Next, measure the length of the bullet, which is pretty straight forward.

Then drop the bullet in the barrel, and measure the depth from the side of the barrel again. Make sure it's the same side as before if they're different.

The just add the bullet length, the bullet depth, and the delta you determined at the beginning. If you have an assortment of bullets, or an assortment of barrels, you can run through them all pretty quickly. It's also worthwhile to do this with your case gage (or gages) to see how far it varies from your bullet. If your gage is looser than your barrel, then it's essentially useless. If your gage is much tighter than your barrel, you at least know that, and can decide whether to just find a more aggresively reamed gage, or just base your OAL on your gage.

The main thing I learned form all this is that I don't think I have ever had a round of 9mm or .45 fail the case gage because of issues with the brass, and the EGW undersized 9mm die I bought was essentially a waste of money. But I've had A LOT fail because of the OAL of the round.

Seriously, if you have 9mm or .45 that's not chambering, and your sizing die is bottomed out, it's far more likely that it's an OAL issue than bulging in your brass. The same applies to issues with the case gage...with 9mm and .45. Now if you're having trouble with .40, that could go either way. But I recently sat down and pulled 30 of the bullets I've had sitting in my "failed case gage" bin, and EVERY SINGLE PIECE OF BRASS fit in the case gage once the bullet was pulled.

Edited June 8, 2014 by Laughingdog

[Hi-Power Jack]

Sounds like a great idea for some people ...

For me, the old Plunk Test works perfectly, so I'll stay with it.

But, thanks for proposing an alternative - nice job. :bow:

[SD1]

nice write up

[kneelingatlas]

For me, the old Plunk Test works perfectly, so I'll stay with it.

+1

[Laughingdog]

The plunk test is better if you're verifying that an existing round will fit in the barrel of a newer gun. But if your gun is throated tightly, like CZ barrels and many match barrels, this is a lot faster for weeding out the ones that would need to be loaded way too short. In 10 minutes, I was able to eliminate 8 bullet profiles for needing to be loaded to 1.100" or shorter in 9mm, and identify the two that work fine (Bayou 124g RN and any Berry's 9mm RN).

[mikeyjones]

This is the method I use :

Take a wooden dowel, insert down barrel of assembled gun. Using a razor blade make a mark on dowel where dowel exits crown.

Take sized case, insert bullet in lightly. The case is only a vehicle to get bullet to touch lands. Once you're happy with bullet contact, insert dowel back into barrel just so it lightly touches bullet. Make another mark on dowel.

The distance between the 2 marks is your max oal. You can subtract a few thousandths for tolerance and voila.

Sent from my ONE A2005 using Tapatalk

[Hi-Power Jack]

The distance between the 2 marks is your max oal.

Mike, that will give you a good approximation, and in many cases possibly the

final answer ...

BUT, it considers only the overall length, and not the shape of the bullet -

sometimes the OAL is fine, but a shoulder on the bullet might snag in the

chamber.

Before you load up a large quantity, you're still better off doing the Plunk Test.

[mikeyjones]

The distance between the 2 marks is your max oal.

Mike, that will give you a good approximation, and in many cases possibly the

final answer ...

BUT, it considers only the overall length, and not the shape of the bullet -

sometimes the OAL is fine, but a shoulder on the bullet might snag in the

chamber.

Before you load up a large quantity, you're still better off doing the Plunk Test.

That's why you would use the actual bullet you're going to load. It eliminates the guess work.

Sent from my ONE A2005 using Tapatalk

[thermobollocks]

I just jam the bullet into the chamber then stick the probe down my barrel until it contacts the tip. Then, remove bullet and get overall barrel length all the way to the breech face.