COL for pistol rounds- what to use?

[Lugnut]

Ok, I recognize that many reloading books recommend or at least have tested a load, bullet, etc at a certain COL. However is there any harm done using a COL that is slightly longer than what a book may post as long at it's not longer than the max? I recognize that one should never go shorter that the recommended COL but is longer ever a problem? I've started doing .40 and I want to make sure I'm safe so I've been going closer to the max COL. On my 9mm same thing... however many books/references tend to vary the COL quite a bit. I find changing the COL for different powders on a 9mm to be tedious and it it necessary if I always err on the cautious side? I realize I might be forgoing some accuracy/speed but I'm ok with that.

 

[Dick]

To the best of my knowledge there's no safety-related reason that you couldn't use a longer COL, provided of course that the round will still chamber properly and will still function through the magazine, which is usually the limiting factor with semi-autos. Also, with the bullet seated out further there will be less friction between the bullet and brass, which in theory could lead to problems with heavily recoiling calibers (the bullet can move in the case, or even come out of the case, under recoil).

You want to be sure that the bullet is not out so far that it's being mashed into the rifling (unlikely due to the magazine limitation, but possible I guess), which will lead to increased peak pressure. If you chamber a dummy round at the COL you want to use and then carefully extract it you should be able to see the marks of the lands on the bullet if that's occurring.

With regard to accuracy, most firearms have greater accuracy when the bullet "jump" is minimized, i.e., when the ogive of the bullet is just touching the lands. I doubt that it makes enough difference for pistol shooters to care, but it's important for careful rifle reloading.

There is also a very poorly understood phenomenon called "detonation" that allegedly can result with small charges of slow-burning powders and is said to cause extreme pressure excursions. That's why you see "minimum" as well as "maximum" loads listed for some powder/caliber combinations. It may be related to the large amount of extra space in the case, and increasing COL would also increase the space. My understanding is that it's primarily a potential problem for rifle cartridges and some magnum revolver cartridges, however. Pistol cases are comparatively small and most pistol powders are not slow-burning, of course, so I doubt that's anything you need to worry about with the cartridges you're loading.

 

[Lugnut]

Thanks for the great detailed response Dick. I haven't have any problems feeding/cycling in my guns at the COLs I've been using. In all cases I've used a COL that is at least slightly less than the MAX for that cartridge so I'm sure my bullets aren't touching the lands.

In some cases where I have the exact same bullet weight as in the reloading manual but I'm using a different mfr (or I'm using FMJ instead of the tested FMJ HP), different primer or different case- I'd just assume be cautious espetially with .40 S&W. I'll use 1.130 COL where the manual might say 1.120 for a "similar" set up.

Seems like I see more variation with 9mm vs .40 S&W. I've seen 9mm with the same bullet weight vary between 1.090" and 1.169" (different loads though).

 

[highlander]

The method dick suggests is what I've done in my semi auto pistols for years and no problems. The only exception are the lead swc I use in my 40 and 45 I seat those bullets so that 1/64' of the top driving band is above the case mouth
then crimp

 

[RKG]

Actually, and I would have to go through an awful lot of old materials to find the specifics on this, I believe there IS (or, at least, CAN BE) an internal ballistic implication of extending cartridge OAL, at least if you go to the limit of boreseating, as suggested.

As I recall (dimly), the nominal OAL cartridge allows the slug to move forward a given increment at much lower mechanical impedence, before encountering the chamber throat and increased impedence. Standard and lab-measured pressures are based on this scenario. Boreseating the slug means that it will start to move later and will a lower rate of combustion chamber volume expansion versus time, and this could well raise experienced peak chamber pressure above nominal peak chamber pressure.