45acp crimp adjustment vs recoil & accuracy

[StrawDog]

I'm curious about how the final crimp affects recoil & accuracy of a semi-auto for a given powder charge and bullet weight. I've been using 4..0gr of Bullseye and 200gr Rainier JSWC. My first notion is that more crimp will contain combustion longer to build more pressure to initially launch the bullet, more of a pop. This should tend to pop open the action of the pistol more abruptly which wouldn't seem desirable. So it seems reasonable to me to lighten up the crimp enough to permit easy launch but not so much that the bullets back out. I initially set up the final stage of a Dillon 550B to give similar impact pull as a factory cartridge, the od at the case mouth is .471". I've backed off on the die 45 degrees and then 90 and produced 20 rounds at each setting. The case mouth od varied from .471" to .473". All were tested with a case gauge. 5 cartridges failed from the 90 degree batch and were rerun through the final stage. (I understand that this is not the best way to use a progressive with a floating head, but they must fit in the chamber) I haven't tried my impact puller test. But I am eager to go shooting. Interested in your comments. Thanks

[That'll Do]

I taper crimp all of my 45 ACP rounds to measure .470" at the mouth after crimping.

A taper crimp (in my opinion) really just removes the flare made to seat the bullet. Bullet pull is more so determined by case neck tension than the taper crimp. In a revolver, a roll crimp definitely plays a part in bullet pull, but not so much with a taper crimp.

[Shoney]

I am a bit confused by some of the terms and explanations you use. Generally, you apply a taper crimp just sufficient to take the bell out of the case. Neck tension of the sized case will hold the bullet in place. In effect you are applying a minimal crimp.

When you apply more crimp to the 45ACP case, you risk the crimp deforming the bullet inward and having the elastic brass case spring back out; and in effect you get a loose crimp. If you indeed get a premature movement of the slide from the heavier recoil, a stiffer recoil spring will remedy that.

Some 45ACP target shooters put a roll crimp on their loads, and say that the resulting increased pressure before the bullet releases gives a more consistent and accurate load.

[StrawDog]

Then it sounds like I am exploring what happens with too little crimp? These appear to be mechanically stable as judged by pressing firmly on the bullet. Also from the ones I've taken apart previously (measuring .471") I haven't noticed any deformation of the bullet, so I don't think I'm over crimping. As far as the gun I'm running a stock spring with a nearly square firing pin stop. Thanks for your ideas

[Unclenick]

Straw Dog,

A couple of points to get on the same page: Loose bullets don't pull in a semi-auto. That only happens in a revolver because recoil transfers to the ammunition via the cylinder which holds onto the cartridges by the rim, yanking the cases off the bullets inertially. In a semi-auto, recoil can push bullets in (bullet setback) by banging the front of the magazine against their noses or when they are pushed up the feed ramp. Nothing is hanging onto the case to pull them inertially in a self-loader. If they are pushed in far enough, it can raise pressure dangerously, so that is to be avoided.

Your Rainier bullets are not JSWC. That would be a jacketed semi-wadcutter, or what used to be called a copper patched semi-wadcutter. Rainier calls their bullets TJ, for total jacket, but that can mislead people into thinking they should be treated the same as a jacket formed from a cup. They should use P for "plated", in my view, but they don't. Here's why it matters: A bullet with a copper jacket formed from a cup is harder to push into the rifling of a barrel than either lead or plated bullets are. Formed copper jackets are actually gilding metal, a very mild brass, being about 5% tin. It is substantially harder than the soft pure copper used for bullet plating. As a result, when a jacketed bullet is forced into the throat of a barrel it takes more pressure than lead or plated bullets to do. Powder also burns faster under higher pressure, so there is a snowball effect, resulting in a faster rate of rise in chamber pressure when you use a jacketed bullet within the normal pressure range than there is with either lead or plated bullets. This affects powder charge.

The other thing that affects charge, but in the other direction, is that neither lead or plated bullets can be driven as hard as jacketed bullets without deforming the bases or increasing metal fouling. Despite producing lower start pressures at their normal load levels, the deformation can, in some chamber shapes, especially revolver forcing cones, actually raise pressure with higher loads by jamming and becoming harder to force into the rifling.

Originally Posted by Rainier's Website

We, at Rainier Ballistics, recommend using lead bullet load data when loading our bullets. There is no need for adjustment when using lead bullet load data. Our bullets are jacketed using an electroplating process and are softer than traditionally jacketed bullets; hence the recommendation to use lead bullet load data. If you only have access to traditionally jacketed load data, we recommend reducing maximum charge by 10%. A roll or taper crimp may be used with our bullets; do not over crimp.

Many years ago, competitors at Camp Perry noticed that military jacketed match ammunition was more accurate than commercial match ammunition. The reason was traced to the pitch seal applied inside the necks of military ammunition before seating the bullet. It sticks to the bullet, raising the pressure needed to unseat the bullet. It causes the powder ignition to be more consistent, so the peak pressure and muzzle velocity are more consistent because the acceleration curve and barrel time are more consistent. The latter two affect how much the muzzle of the gun is elevated at the moment the bullet exits, and thus affects vertical stringing. It also affects the torque that turns the pistol a little in reaction of the rifling against the bullet, which affects horizontal point of impact.

50 years ago, target shooters would roll-crimp a .45 ACP about 0.020" to 0.050" below the leading edge of a lead bullet's bearing surface. (Taper crimps, which prolong brass life over a roll crimp, had not been invented or at least were not common.) The plated bullets are soft enough that they would probably have tried to do the same with those if they'd had them at the time. The roll crimp is to increase bullet release pressure. That leads to more consistent barrel time, as described below.

You might, at this point, ask whatever happened to headspacing on the case mouth? 50 years ago the target shooters didn't. I've seen estimates given that up to 70% of 1911 style pistols chambered for the .45 ACP are actually headspacing on the extractor hook, anyway. Their chambers are often too long to headspace on a case mouth before the extractor hook stops the cartridge from going further forward. Additionally, though, when the bullet sticks out a little from the end of the case, the cartridge can headspace on the bullet stuffed into the throat. In either instance, the exact diameter of the edge of the case mouth becomes irrelevant. At that point it isn't being used as John Browning intended, anyway.

In revolvers, the roll crimp hooks the bullet to keep it from backing out under recoil. In pistols, the taper crimp is supposed to form a slight crimp groove to stop bullet setback. In practice, jacketed bullets have enough friction with the brass they don't need a crimp to prevent setback. As a result, most jacketed bullets in .45 ACP get no true crimp, and in reloading and the flare is just taken out.

At the opposite extreme, cast lead bullets with an external lubricant can sometimes be seated deeper into a case by thumb pressure alone. These guys need the taper crimp to form that slight crimp groove to prevent setback. That's why a taper crimp isn't just a straight cylinder, which is all you need to remove a flare. The taper, however, has little strength against forward bullet movement, so it doesn't add a lot to start pressure, the way a roll crimp does. It's only to prevent setback.

So, what's a fellow to do? First, note that changing the crimp is a moderate exercise in pressure change. It's only on the order of 2-5% or so. You have a load that is running at around 10,00 psi in a gun rated for at least 21,000 psi (or 23,000 psi for +P loads), so that small increase is meaningless as far as gun damage goes. The importance to accuracy is the crimp gets the early part of the burn to be more consistent. Doing that basically just requires crimping hard enough so the case lets go of the bullet at the same pressure every time and so the bullet isn't easily unseated by the primer before the powder has a chance to light up. Such unseating tends to be erratic and messes with the barrel time. Fast powders have plenty of time to catch up and burn, but needing an extra millisecond to do it can put the bullet on paper at a different location.

So, what you really want is just a way for the bullet to stay put long enough for powder to burn consistently. You also don't want soft bullets to go into the throat cocked at an angle, as headspacing on the extractor hook does. A jacketed bullet can usually straighten out from that small tilt and do OK. Soft bullets are deformed by it and wobble slightly in flight. Not a good for accuracy. The best solution I know for self-loaders is to intentionally headspace on the bullet every time. The bullet goes into the bore straight. The start pressure is consistent for the powder burn. You still need a taper crimp to prevent setback, and 0.467" at the case mouth is the SAAMI minimum spec even keeping to Browning's headspacing method, but you don't also need a roll crimp, so your brass will still have longer life. How to set up to headspace on the bullet is shown below, third from the left.

[StrawDog]

Unclenick,

Thanks for the extensive explanation. I appreciate the corrections. I haven't tried my experiment yet. Question: if 0.467" at the case mouth is the SAAMI minimum spec, what is the desired nominal? Sounds like length is important. I'll remove my barrel and use as a gauge to lengthen OAL.

[crimsondave]

Unclenick, ur still the man. Even when you post about something I think I know about, I learn something new.

[WESHOOT2]

I've found .470" to be ideal.

[Unclenick]

Strawdog,

.45 ACP tends to shrink with each reloading cycle, so it doesn't normally need trimming. However, getting the trim length the same among your cases can help keep crimp tension uniform, especially with a roll crimp. For taper crimps, brass thickness differences will alter the crimp tension more than length variation. I buy and keep the same lot of brass together for target shooting accuracy for that reason. For combat accuracy, I let the cases mix and match, rejecting only steel and R-P. (That is a habit I developed in the 80's because R-P (Remington-Peters) brass always had a thinner wall at the mouth than other brands in .45 ACP. That would work-harden and refuse to give any neck tension after a couple of load cycles. I haven't looked at it for a long time, though, and don't know if it's changed since then? I understand Remington contracts a lot of their stuff out these days.)

If you headspace on the bullet or the extractor hook, you are not headspacing on the case mouth, but are stopping the cartridge going forward in the chamber before the mouth of the case gets to its counterpart in the chamber. This has to be done when roll crimping, as the the previous generation of bullseye match shooters did. So, the diameter isn't critical in these cases, as long as the crimp bites lead bullets enough so they can't get pushed in by either recoil or chambering.

If you are headspacing on the case mouth, then the mouth diameter needs to be somewhere within the 0.006" spec range. In that instance I would aim for the middle ground of 0.470" as Weshoot2 suggests, and expect a thousandth or so variance up and down. Be careful that a case brand with a thin wall at the mouth still has the bullet impressed enough to prevent setback. You might have to aim at the low end of the spec with those, depending on your bullet diameter?

[briandg]

You mentioned that you were concerned that the longer time with the cartridge and bullet intact would cause some potential harm to your pistol when the thing unlocks? at least that is what I understood.

This should tend to pop open the action of the pistol more abruptly which wouldn't seem desirable.

This is not a concern. Barrel and breech are locked until that firearm is well into full recoil; I believe that your bullet will be out of, or nearly out of the before the barrel begins to disengage. Chamber pressure should be -0- by the time the thing is ejected. that is why the things are built with the linkage.