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The Physics Of Reloading

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#1 Vlad


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Posted 04 February 2004 - 09:49 AM

Hi folks,

I though I mostly understood the physics involved in reloading and what affects what. However I have been reading some of the older threads and now I am a bit confused.

For the scope of these questions, I am going to use my current 9mm loads as examples. I am currently loading 147gr Zero's over 3.4gr TG, OAL 1.155.

1) Is the relationship between powder charge and bullet speed close to linear in the %10 charge interval before published max (assuming all other variables stay constant). I assume that the velocity is a function of pressure, so the more important questions is how curved is the curve of pressure in those last %10? Is a linear approximation close to accurate? This is somewhat important to me as I don't own a chrono and I try to estimate velocities.

2) How much of a difference does barrel length make? Most published data I saw uses a 4" barrel. I shoot a CZ with a 4.75". In rifles, you can often lose or gain 100fps per inch. How does that translate for handguns? I see loads posted by people here, which wouldn't make minor using the published data for 4" and assuming a near linear curve as in (1). For example someone mentioned a minor load using 3.2gr of TG under 147gr in a G34. That would be 121PF out of 4" barrels. However the G34 has a 5.3" barrel. Is that why such a load makes minor?

3) How much velocity difference does bullet design make? Do hollow points move slower or faster then FMJ? Lead and plated are in their own land, but how about different designs of FMJ, like RN of flats or SWC?

4) Seating depth vs velocity, whats the story? Note that earlier I said that my loads are 1.155, which is quite long, but my CZ has no problems with them. However the TG data is for 3.6gr at 1.1 OAL. Obviously going too short can create pressure problems, and my current load seems to be under-pressure as I get soot about 1/3 of the way down the case. I load as long as I can, because I have no idea what the actual sizes of bullets used for testing are. To my thinking the external length is only part of the problem. Bullets with different designs (HP vs RN) will most likely have different length for the same weight. A such the OAL tells me nothing about the volume inside the case. Should I load shorter to get higher velocity out of less powder? Go until I see pressure signs, and then back down?

5) How much does crimping affect pressure? Where does one get information about how much of a crimp is to much, how little not enough?

6) How does load data get decided? I was quite confused about the data for TG and 147's in the Hodgdon manual. The max load they list is 3.6gr for a pressure of 27,500 CUP. I would think that is peak pressure, or otherwise the data is meaningless. However other loads they list go as high as 31,000 CUP. Nevermind the SAMMI 35,000 PSI listing and the CUP vs PSI discussion. Lets assume that for lawyer reasons they decied to not publish anything over 33,000 CUP (They have some loads close to that for 124gr). Why stop at 27,500 for TG and 147s? Not that I have any need to go beyond it, but I am curious?

Sorry about the size of my post. I supposed a lot of these could be answered by buying myself a chrono (it's on the list, but somehow there is never enough money for the the whole list) and doing a lot of testing. However I figured others have already done most of the work, so lets ask questions first.


PS: There are partial answers to these questions here and there in the archives, but often the contradict eachother.

#2 RogerT


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Posted 04 February 2004 - 10:54 AM

Hi Vlad and Welcome to the land of the occult..I mean reloading...

1. Velocity is a function of pressure and the curve is NOT linear, how steep the pressure curve is differs between powder types and charge volume. There is no way of guessing velocity with any accuracy, to many variables.

2. In handguns the burn rate of the powder is so fast I don't think barrel length has that much of importance, but barrel construction and groving might.

3. Bullet design in handguns does very little in speed difference compared to rifle bullets.

4. Yes.

5. Trial and error. Lead bullet crimping is very important to be consistent, jacketed bullets I don't know.

6. Manufacturers of powder try out loads and publish the results. There so many variables involved, there is nothing but max pressures set as standard (or nearly standard..)

Any one else with an opinion?

#3 shred


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Posted 04 February 2004 - 12:02 PM

2. In handguns the burn rate of the powder is so fast I don't think barrel length has that much of importance, but barrel construction and groving might.

Barrel length does matter.. somewhat. More so with slower powders and larger charges.

For example, the same 5.2 Titegroup/121 HAP load that does ~1200 fps in my shorty Super does ~1400 fps in my 16" barreled CCU.

4.1 N320 / 121 HAP doesn't seem to care if it's in a 4" or 5" Super barrel (~1020 fps for both), but goes 1140 fps in the 16" bbl.

A slower powder like N350 or 3N38 shows variation not only in the short/long pistols (1360/1395 fps respectively), but also in the CCU (1620 fps!)

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#4 RogerT


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Posted 04 February 2004 - 02:22 PM

OK Shred, you'r right of course, but Vlad loads 9mm and from what I've heard at the club it's a very tricky ammo to load and get god results. Sensitive to small changes on any of the involved parameters. The .45 on the other hand is the opposite, right?

#5 TBF


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Posted 04 February 2004 - 08:08 PM


45acp may be the most " user friendly " cartridge known to man.
Dunno about 9mm, but some of the other pistol stuff I load is pickier.


1. No, but with some loads it's close, some of the time, but don't count on it.

2. Size matters, with slow powders , it matters alot.

3. Bullet construction affects seating depth, and thus pressure. Hollow point bullet will be longer and also due to the profile need to be seated deeper . This puts the base of the bullet deeper in the case. The distance of the bullet base from the case head being equal , most jacketed bullets would behave similarly , with the same pressure.
It would be nice if instead of OAL, they gave base to casehead measurement, but that might confuse some people.

4. Go with what feeds, adjust powder accordingly.

5. In auto rounds, just take out the bell.
bullet dia. + 2X brass thickness - .001 - .003 ( this is not written in stone )
Revos with slow powders need a firm roll crimp to ensure ignition.

6. I dunno for sure. Some powders are at the extremes of their pressure curve in some loads. The guys with the expensive machines set the limits based on their results. Some slow powders are dangerous when loaded at lower than design pressures. Some fast powders are dangerous when loaded at higher than design

7. You didn't ask, but yeah, some posts contradict other posts. We all relate our own perception of our experience. We interpret results differently. And sometimes jump to conclusions based on what seems logical , without doing the research to validate our ASSumptions. Nobody knows it all , thats why we come to this forum.

As with any advice, YMMV.

Travis F.

#6 bigsaxdog


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Posted 05 February 2004 - 07:25 AM

i've been screwin' around with .45 loads, trying to get the softest major load possible. there's so many parameters involved with every change of components...i've been bogarting my friends chrono, and have learned that it's really a necessary piece to get a load together. you really learn alot when you start watching the velocities. my 5" gun is totally different from the 6" gun, and who knows why? powders, bullet styles, barrel lengths all make signifigant differences. you gotta get a chrono, or bogart a buddies.......... :blink:
excuses don't win ball games!

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