Start Pressure

[maverick5582]

Has anyone actually measured the start pressure with the side crimp?

I use a QuickLOAD program and I am assuming from what I have read that the side crimp pretty much works like jamming the bullets into the rifling in the barrel.

John

[pitted bore]

maverick5582-
That's a really interesting question.

I wrote a few hundred words in an attempt to answer it, but then decided that I didn't clearly understand your point.

Below is an generalized image of a pressure curve. What part of the curve is "start pressure"?

Your concept may be innovative and valid, and I'd appreciate help in understanding it.
Thanks.
--Bob
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[Hoot]

The fellow who got me started with QL and who had at first did workups for me in the 450b and then crunched my actuals versus QL's predictions, told me that I would get more accurate predictions when loading lighter bullets (<=230gr), using the taper crimp in this caliber, by using a start pressure of zero. Once I got my own copy and did my own workups prior to shooting, this proved to be correct. I assumed it was from the challenge of getting good neck tension using just the taper crimp.

With the side crimp, stab crimp, LSC, call it what may, tension is proportional to the degree of side crimp you use, so I doubt there is a specific start pressure that serves all side crimps across all bullet weights. Kind of a sliding scale.

Over the years, I have found that in the case of extremely aggressive side crimps or moderate ones into the driving band grooves in Barnes bullets, you will actually exceed the predicted resulting velocity of QL using the standard start pressure it automatically ascribes to jacketed bullets.

IMHO, there is no single, magic number to put into the start pressure for this caliber. I'm guessing that's what Bob was going to say in his longer response.

Hoot

[maverick5582]

Bob: Looks like Hoot and I were typing at the same time. Thanks Hoot. I will use a start pressure of zero with QL.

[Hoot]

I forgot to address your comparison of the side crimp to loading into the lands. While I have never compared them closely, I'd say that is probably a good comparison. I actually did an experiment with one of the 300 grainers that actually touched the lands in my particular chamber at 2.22 COL. I was trying to get AA1680 up to full power before the bullet left the barrel and like you, figured loading them to the lands was as good a source of resistance to movement as a tight crimp. It was, but the weight of the bullet being what it was, it still went down the barrel too quickly and exited without really getting the 1680 going full tilt. We really need a powder to fill the gap between W296's burn rate and AA1680. IMR4227 tries but does not have the energy content we're looking for. I recently did some experiments with Alliant 300 MP, which while a little slower and mean just a little, than W296, was still too fast for the 300s. It does do better than either Lil Gun, H110 or W296, but the signs I got from it were indicative of too fast. Let me see if I can find that thread on the rounds I loaded to the lands...

Found it here

WRT the results of that experiment, 300s from a 20" barrel should safely reach 2100+ fps and need that and more to stabilize and tighten up the groups. Didn't quite get there with 1680. As always, it's fun trying these off the wall ideas out. We really need Hornady to produce H4227 again. When they acquired IMR, they dropped their version and went with IMR's. Don't believe it for one minute when they tell you they are identical. One is cotton lint based and one is wood lint based. It matters.

I'm not out of 300 grainers yet from the last ones I bought, but I'm about out of 300 MP. One of the angles I missed in testing 300 MP was to try cooler running primers to perhaps slow it down a little. That experiment is in the mill. Some experiments are altruistic. The 300 grainer research is one of them as I don't need or care for them in the 450b for what I hunt up here in MN. Way too overkill.

Hoot

[maverick5582]

I wish I had that many choices of powder to work with. My powder supply is a very sore subject with me these days. Had I know what was going to happen with the DHS attack on the powder supply I would have mortgaged the house and bought all I could have found.
QL has opened my eyes to all the variables I never knew existed in my study of ballistics. Some I have figured out and others still remain on the dark side for later investigation. I have a sonic cleaner now and I repeatedly clean my brass and tumble it to get all carbon out of it I can. When I cut the neck off two of the old 6.5x284 cases the other day I thought maybe they will be easier to clean the inside of the case with the 45 caliber hole in the front.
I shoot mostly paper and hunt whitetails so I am thinking that bullets in 225-250 grain range will cover what I want to do based on my reading here. While running some what-ifs on QL I noticed that all the loads show 100% burn with 20-24 inch barrels. I noticed only a 78 f.p.s. loss in velocity from 20 to 24 inches of barrel length. So the 20” barrel seems to be the winner.
Unless something better comes my way I am thinking about a McGowen barrel blank. Does the 1:24 twist rate seem the best for the 225-250 grain bullets? Does everyone start out with the Hornady .452 diameter bullets or go to the .451 pistol bullets?
When I cut the necks out of the 6.5 brass they are 1.775” long. I had thought about 1.700” on the chamber, but I guess I can go farther than that based on what you said about brass shrinkage. If the Lapua brass needs to be reamed that should not be a problem.
From what I have read the mid-length gas system seems to be the standard on a 20” barrel. I don’t know much about that as I had a standard measurement of 12.140” from the barrel extension to the gas block shoulder when I made the Grendel barrel. I peck-drilled the gas port with a .099” bit. It cycles fine with that arrangement.
I said all that to say this, any suggestions you have would be appreciated in helping me to decide on which way to go based on your experience with the 450. I really do not want to go into uncharted territory on this build.

[Texas Sheepdawg]

If I'm not mistaken, the 20" and 16" are both carbine length gas systems.

[pitted bore]

maverick5582-
I'll make a first attempt at answering some of your questions. Those that I omit, and those that I mess up, will be fixed by the pros who will show up in a bit.

I wish I had that many choices of powder to work with. My powder supply is a very sore subject with me these days.

Unless you are planning on loading bullets more than 300 grains, or less than 240 grains, the most useful powder is Lil'Gun, with 296/H110 running a close second. Lil'Gun powder is again becoming available at retailers. Last week I received a few lbs of Lil'Gun from a reputable dealer who has maintained prices through the present problem period. If you'll PM me, I'll supply info. (I may or may not respond to others who might inquire.)

I shoot mostly paper and hunt whitetails so I am thinking that bullets in 225-250 grain range will cover what I want to do based on my reading here.

The available tested data from Hornady for their 240 XTP-Mag, the 250 FTX, and the 300 XTP-Mag bullets (or their close equivalents from other mfrs) will handily and conclusively whack any piece of paper or any whitetail.

Unless something better comes my way I am thinking about a McGowen barrel blank. Does the 1:24 twist rate seem the best for the 225-250 grain bullets?

I may have been the first to purchase a McGowen barrel for use with the 450B. My gunsmith, a picky SOB, seemed happy with the quality when he installed it on my bolt action. The barrels come in 451 bore diameter, but that should make relatively little difference. The 1-in-24-inch twist is adequate for all but really long custom bullets. As you know, 1:24 is the twist rate of the original rifles, which were noted for fine accuracy.

Does everyone start out with the Hornady .452 diameter bullets or go to the .451 pistol bullets?

Although the official specifications for the 450B call for 452 dia bullets, they also specify tolerances that allow bullets with diameters somewhat larger and smaller than 452. Some of the bullets I've played with, and some others that are on the agenda for work this summer, have diameters of .449" or less. The taper crimp die can deal with these, and one or another of the collet crimp dies can be used if necessary.

The gas gun gurus will be along shortly with advice on managing the operating system.

Good luck, and keep the forum informed as you encounter difficulties and find techniques for dealing with them.

--Bob