How to decide on 9mm COL?

[cdoc42]

I bought 115gr JHP bullets from Precision Delta and I have to figure out what the COL should be. I have Hornady 115gr XTP on hand and they are 0.541" long. The PD bullets are 0.551" long.

The 50th Lyman recipe for the XTP calls for a COL of 1.090". But Lyman also shows a Barnes TAC-XP 115gr having a COL of 1.100."

Hodgdon data shows a 115gr Speer Gold Dot HP COL as 1.125" with Hodgdon powder but with Alliant they list a 115gr FMJ, no manufacturer COL as 1.120."

Hodgdon also lists the following COLs for a 147gr Hornady XTP:

with Hodgdon powder, 1.100"; with Alliant, 1.140"; with IMR 1.130"- all with 4" barrels? In fact, reviewing bullets weights from 90gr to 147gr there is no agreement among Lyman, Hodgdon, or Speer data.

Speer lists a COL of 1.125" for their 115gr Gold Dot HP; 1.135" for their 115gr TMJ and 1.125" for their 115gr JHP.

How does one figure out what the seating depth and COL length should be for a generic JHP-HP?

[74A95]

First, determine what length is required to fit your chamber. If you have multiple 9mm pistols, you might need to check them all. You can determine this by doing the 'plunk test'. Google search for it.

Once you know the length to fit in your chamber, subtract .010-.020" to ensure clearance of the riflings. Make sure it's below the maximum OAL for 9mm, which is 1.169".

Then you have a free hand for what OAL you can use. It must: fit in the magazine (ensured by staying under 1.169" for round nose and under 1.150" for flat nose bullets) and feed reliably.

Then use the load data as your guide. You can load them to any OAL you want as long as they fit the above criteria, and if you load them deeper than what is shown in the load manuals, you have to reduce the powder charge to keep the pressure the same.

[44 AMP]

How does one figure out what the seating depth and COL length should be for a generic JHP-HP?

Easy, shorter than the listed max length of 1.169". Enough shorter that they feed from your magazine(s) from bottom to top, and into your chamber without hitting the rifling.

The rest is ...variable.

Its helpful to understand a few things about varying COL. COL is measured from the base of the case to the tip of the bullet.

The bullet seating stem does NOT contact the very tip of the bullet. It bears on the ogive (sloped side of the bullet) at a certain point, determined by the width of the bullet at that point. From that point forward to the tip of the bullet, variations in length are largely irrelevant provided the COL is still under max length.

Put another way, you can have half a dozen bullets from different makers, or different styles from the same maker, and have them ALL seated so the bullet base is at the same depth in the case, all having the same amount of powder space, but the bullet TIPS can be different lengths, and that is what you are seeing with different COLs, because its measuring to the TIP of the bullet.

Just look at a 115gr FMJ 9mm bullet. Most have a long pointy nose. Now look at a 115gr 9mm Hollow Point bullet. Its shorter. When you seat both to the same depth in the case, the hollow point will have a shorter COL than the fmj round. It's normal.

The main reason you see different COLs for specific loads is because they each use a different bullet, and because of differences in the nose profile and length of the bullet, that's what the COL turns out to be.

You can even find variations of length in the box of bullets on your loading bench. Go measure some and see. Expect very close, but there will be differences, small, but measurable.

[TailGator]

When changing bullets, I think about the size of the combustion chamber, the space in the case behind the bullet. It is going to be pi times the square of the radius times the length of the cylindrical space. Pi times the square of the radius won't vary, so the only variable is the length of that space. It is going to be the COL minus the bullet length. If you keep that consistent, within the same bullet weights you will keep your pressures consistent, within reasonable limits of measurement and variation.

[cdoc42]

So what I should be after is the same size space behind the bullet available to hold the powder.

So if the 115gr XTP data shows a COL of 1.050" and my 115gr XTP bullet is 0.541" long, 1.050 - 0.541 = a powder area of 0.509".

The new 115gr bullet is 0.551" long. 0.551 + a powder space of 0.509" = a COL of 1.060".

Does that make sense?

[rg1]

One thing I do when changing same weight bullet brands or styles is to take a round I previously loaded that chambers perfectly, is to place one of your 115 Hornady rounds in the shellholder lower your seating die to get firm contact with your die touching the bullet, lock the lock ring. Now seat the new bullet and measure the oal. Gives you some idea of a good oal. The die will touch the nose of each bullet at the same diameter and should chamber with nearly the same distance from the lands of your barrel. Also helpful when changing from FMJ-RN to HP and vice versa. Just interesting and a helpful step.

[cdoc42]

rg1, I understand the principle of your procedure, and it would seem to make sense. If a 115gr Hornady XTP functions in my gun at a COL of 1.050", loading any new 115gr bullet to 1.050" should replicate that function. But my concern is that all 115gr bullets themselves are not the same overall length, so as described above, a longer bullet seated to the same COL leaves less space in the case for powder, and I assume one would use the same charge for the same weight bullet, so with an increase in seating depth, pressure will rise as should velocity.

As such, it seems prudent to keep the same case volume behind the bullet, regardless of its overall length, for the powder. That's what I think my calculation above achieves. It also explains why all the COLs in the data bases are variable. Caution would be applied, however, to be sure the longer bullet avoids being jammed into the rifling. I would be interested in the opinions of those who have participated thus far (as well as anyone new).

[74A95]

Originally Posted by cdoc42

But my concern is that all 115gr bullets themselves are not the same overall length, so as described above, a longer bullet seated to the same COL leaves less space in the case for powder, and I assume one would use the same charge for the same weight bullet, so with an increase in seating depth, pressure will rise as should velocity.

Yes, the general rule is that a longer bullet of the same weight seated to the same OAL will increase pressure.

But, bullets vary with respect to several things, so don't assume they will react with the same pressure even if of the same weight, length and loaded to the same depth. Differences in bullet/jacket hardness and the length of their bearing surface vary and these can affect pressure. In other words, don't assume too much.

Here is the universal rule of handloading; work your load up from the starting load and ensure it is safe in your gun.

Originally Posted by cdoc42

As such, it seems prudent to keep the same case volume behind the bullet, regardless of its overall length, for the powder.

You can do it that way, but you don't have to. After 45 years of handloading, I never have, and I suspect few people do. It can be a useful rule of thumb for some folks, but the best results might not follow that rule. A different OAL might be more accurate or feed more reliably with bullets of the same length, and see my previous comments.

There is a lot of flexibility in handloading. Follow the safety rules, but feel free to experiment.

P.S. It's good that you're thinking about it. It will help you stay safe.

[44 AMP]

74A95 hit on the other factor, the one that's usually left out when people start worrying about deeper seating increasing pressure.

There are two parts, one is how much higher pressure a small change in the available volume makes, and does it matter???

because, if you are following proper procedure loading you will be adjusting the powder charge for the available space, anyway.

You should be reducing the load, because you changed bullets, and then work back up to find the new load for the new bullet (it may be the same, it may not be), you won't have any overpressure situations, Same process should be followed when you change any component.

Its not just the length of the new bullet that matters there are other factors like length of bearing surface and hardness of the jacket or lead alloy that have an important effect on what pressure is needed to move the bullet down the barrel at the desired speed.

[Aguila Blanca]

Originally Posted by rg1

One thing I do when changing same weight bullet brands or styles is to take a round I previously loaded that chambers perfectly, is to place one of your 115 Hornady rounds in the shellholder lower your seating die to get firm contact with your die touching the bullet, lock the lock ring. Now seat the new bullet and measure the oal. Gives you some idea of a good oal. The die will touch the nose of each bullet at the same diameter and should chamber with nearly the same distance from the lands of your barrel. Also helpful when changing from FMJ-RN to HP and vice versa. Just interesting and a helpful step.

This achieves the same C.O.A.L. as the first bullet, but it completely ignores the fact that the new bullet may be -- and probably is -- a different bullet length, which (as cdoc42 has commented) changes the available case volume behind the bullet. And that changes the pressure developed when firing the new bullet.

The real answer is that when you change anything, you need to go back to square 1 and work up the load again from scratch.

[cdoc42]

Just to update. I had Remington 9mm JHPs from a bulk purchase I made in the 80's that I seated at 1.050 COL with 5.7gr of CFE Pistol (max = 5.9gr). These finally ran out and I now have Precision Delta 9mm in 115gr that are, as noted above, slightly longer. So I did what I discussed above and seated them at 1.060", still using 5.7gr of CFE Pistol. No problems at all with chambering or firing, extraction, etc. The only difference I can relate is the point of impact was about 3 inches higher at 15 yards than the Remington bullets.

[Aguila Blanca]

Three INCHES higher at 15 yards? That seems like a lot. Same bullet weight? If you have any of the originals left, can you compare the velocities?

[cdoc42]

Same 115gr weight. Longer bullet but only by 0.01"- recall I seated at the same powder volume and powder charge 5.7gr.

I have originals left and access to a chronograph. When time permits I will check it out.

While I am here... I was, at one time, loading a lot of .357 Mag and I accumulated 4000 CCI 350 Small Pistol Mag primers. Any reason why there can't be used in .38 special or 9mm?

[44 AMP]

CCI 350 Small Pistol Mag primers. Any reason why there can't be used in .38 special or 9mm?

Some say mag primers raise pressure by (as much as) 15%.

Others say something different.

I wouldn't use them in ,38 or 9mm simply because its a waste, and I have .357s in plenty, so I'd have a use for them.

Sell or trade them to someone who can use the extra.

There's no mechanical reason you can't use them, just work up loads with them