|August 2, 2011, 08:48 PM||#1|
Join Date: March 20, 2010
Location: Raleigh, NC
Effect of shorter-than-recommended OAL
I'm working up a new load for my Keltec P-11 with my own cast 125gr lee truncated cone TL bullets. I've run the loads through a .356 sizer and they still will not chamber in the gun at the recommended OAL (1.15, according to Lee). At this point I'm running a light charge(3.8 gr BE) well below minimum at 1.12", but feeding is still iffy. I think I would like to load it down to around 1.10-1.11" for maximum reliability. Now, with a charge that's a full grain below what's listed, I know pressure is not an issue, so what do I have to lose with this short OAL?
As it stands, I can only think of two things:
1) Less case capacity, smaller charge (to maintain safe pressures), and as a result, less velocity. In other words, total propellant energy is not as high as it could be, so there's less total energy pushing the bullet at the same pressure you'd see with a longer oal and increased charge.
2) Less inherent accuracy. If the chamber is long enough (I imagine it is, and my chambering issues are related to bullet design, not chamber length) then the bullet has a longer jump to the rifling, so accuracy will decrease. In a P-11, this is no big deal.
Am I missing any disadvantages that I might incur as a result of this short OAL loading?
|August 2, 2011, 09:46 PM||#2|
Join Date: January 6, 2009
I am getting too old for this CrPP. You are creating a second problem (maybe) in trying to correct your number one problem. Solve that one first then go on to the second (of course the second will not exist if you solve the first.)
Number one, have you made a dummy round with your cases and bullet. No primer and no powder. This is where you should start first. Then find out why your 125gr lee truncated cone TL bullets are not working at a OAL of 1.15.
My first guess is that the bullet needs to be swedged more, the 9mm case is a tapered case, smaller at the mouth than the base (head). Standard 9mm bullets (jacketed) are .355 If these fit and feed without any problems than that's your problem, the width of the case and bullet inside the case is too large for your chamber. Does not have to be a lot, but if in a new gun then it will need ammo that is more to spec.
Second take a resized case by it's self and see if it will fit in your chamber (should if your brass is not too long), But if you flared the case mouth to get your cast bullet into the case without shaving any lead off the bullet, you need to check that you crimp the case just so slightly that the mouth of the case is pushed back to were it should be to feed properly.
If you are getting line and groves on the bullet at 1.150 you sould be OK since the case indexes on the mouth of the case. As long you are going to full battery you should not have any problems with soft lead. However I would not do that with jacketed bullets that require more energy to get started in the barrel that would create excesesive pressure.
When all else fails go to jacketed bullets and get a different set of molds with a bullet that will fit your gun.
Si vis pacem, para bellum
Last edited by Jim243; August 2, 2011 at 10:02 PM.
|August 2, 2011, 09:51 PM||#3|
Join Date: December 26, 2004
Location: Louisville KY
That Lee mold will hit the leade early...I already tried it in my CZ.
I settled on a Lee 358-125-rf mold and had to use a 1.01"oal before it would chamber. Just use a minimum charge, should work fine.
|August 3, 2011, 11:12 AM||#4|
Join Date: March 4, 2005
CAUTION: The following post includes loading data beyond currently published maximums for this cartridge. USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assume any liability for any damage or injury resulting from use of this information.
The short seating depth happens with a number of TC bullet designs, including the jacketed ones; they have to be loaded short. You also find occasional foreign-made cases that have thicker walls than average and that were actually loaded with slightly undersize bullets originally. If you have doubts, the use of the inexpensive Lee Carbide Factory Crimp die can help ensure feeding.
As to the load, the shorter seating doesn't always increase pressure as much as an internal ballistics program suggests it should. That's because, in a short case with a small powder space, the primer often gets the bullet started out and into the lands before the powder peaks. But you can't assume that.
One strategy I've used for shortening COL that tends to be safe (when shortening, not when increasing, as it errs on the small side for shortening, but on the large side when lengthening) is to keep the percent of powder space filled under the bullet constant. Though it errs small, the primer contribution isn't changing, so it seems to be a good starting point. To do this takes a couple of steps. First, find out the seating depth of your bullet at the recommended COL and at the new COL. Seating depth is how far below the case mouth the bullet sits.
Seating Depth = Case Length + Bullet Length – COL
Now figure the volume of water capacity in grains that your bullet occupies below the case neck.
Where all dimensions are in inches:
Bullet Seating Volume (in grains of water volume) = Seating Depth × π × (Bullet Diameter / 2)² × 252.9
(multiplying by 252.9 converts cubic inches to grains of water volume.)
Now you need the case water overflow capacity. Take the case you've been using for measurements and plug the primer pocket with clay. Weigh it. Fill it with water level with the case mouth (no meniscus) and make sure there are no bubbles inside and no stray water drops outside. Weigh it again. Subtract the first weight from the second. The result is called case water overflow capacity, even though you don't actually want it to overflow, as you want no meniscus. You can multiply the result by 1.003 to adjust for typical water density at room temperature.
Case Water Overflow Capacity = 1.003 × Filled Case Weight - Empty Case Weight
Subtract the the two Bullet Seating Volumes you figured out earlier from the case water overflow capacity. This will give you what is called case water capacity (this is not overflow capacity; just the capacity under the bullet) for each bullet position.
Case Water Capacity = Case Water Overflow Capacity - Bullet Seating Volume
Divide the smaller Case Water Capacity result for the short COL by the larger result for the Long COL. The result will be less than one. Multiply your charge for the longer bullet by this fraction to get the charge for the shorter seated bullet.
Long COL charge × (Case Water Capacity with Short COL / Case Water Capacity with Long COL) = Short COL charge
Use the same method for both starting and maximum charges to give yourself a prospective charge range. This tends to be pretty accurate in a large rifle cases, but in a short pistol case it tends to err on the conservative side as long as you start with a longer COL's load range. Don't go the other way (short COL charge to long COL charge) or you will get excessive pressure.
Good luck with it.
Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Rifle Instructor
NRA Patron Member