Low Pressure Loads

Hi Bruce.

Once again I have been pondering great thoughts and need to go to the mountain (you) for answers.

Considering reloading data indicating chamber pressure in PSI, (not LUP for now…) what are your thoughts on the following:

1. Pressure in relation to felt recoil. Given that physics must be satisfied, the amount of recoil is the same, but… with lower pressures, do you deliver it over a longer period of time and does this give the push recoil versus the sharp recoil?

2. Shot deformation… With less pressure, is the amount of shot deformation less (this is good, yes)? And would it be significant? Sounds like a job for the patterning board. At what yardage would the “flyers” or deformed shot start to really show up at either missing from the patterning board or outside of the effective pattern?

3. Shooting low pressure rounds through a gas gun, my faithful Beretta. If we have a slow burning powder producing less pressure, will the bleed off gases have a greater effect on the velocity at the magic distance in front of the barrel? I guess I am thinking that the powder is burning most of the way down the barrel and that the bleed off could change that burn by lowering the pressure due to the bleed off. I would guess if I ran some samples of the same reloads through an O/U with the same length barrels as my auto, I could see in relative terms what kind of loss it suffers.

For example shoot 5 fast powder shells through the O/U and then the auto through the chronograph. Look at speeds in ratios then for a relative measurement (auto speed/over under speed). Then repeat the test with 5 slow powder/low pressure rounds through each, and then look at the ratios. Will they fall closely together or make a big change. Assume same FPS out of the reloading data, and hand measure powder and shot to keep it really close. Same hull, wad and primer combination if possible.

What do you think? Any experience with this kind of information?



Dear Jim,

This has to be sort of short because the sun is out, the sky is blue and my wife and I are going shooting as soon as I hit the “send” key.

Bottom line first: Yes, I think that you are technically correct in all three of your assumptions. No, I don’t think that the difference is enough to matter or be noticed in most cases. Not fudge words “most cases”.

1) This is a common question- “Do low pressure loads kick less than high pressure loads, all else being equal?” In theory, the low pressure load gets its velocity by maintaining pressure for a longer period of time, while the high pressure load gets it all at once. If you graphed the curve on a time line/pressure basis, the areas under the curves would be about the same, but the low pressure load curve would look sort of wide and flat, while the high pressure curve would look more like a traffic cone or witches hat. Please note my apt scientific phraseology.

When you look at the curves, it would seem that low pressure gives more of a shove than a jab and would feel like less kick. If you’ve shot any black powder guns, you will certainly notice the difference when that very sloooow black powder recoils. It’s much more of a push than a poke compared to smokeless. But in the real world of smokeless powders suitable for target loads, the difference in burn rate times is measured in such small portions of a second that most people can’t feel the difference. Years ago I did a blind test on some shooters using two loads with identical muzzle veolcities. One was a standard “fast” target powder and the other was two grades slower from the same maker. Half the shooters thought the fast powder kicked less, half picked the slow one.

Another thing to remember that it takes more grain weight of slow powder to equal a fast powder. The weight of the powder charge is included in the formula for free recoil. More powder causes more mathmatical recoil. Not much, but some.

2) Shot deformation and powder burning rate is definitely a situation for the pattern board. In theory again, a slower burn rate would seem to mean less abuse for the shot, less distortion and fewer flyers. One of Don Zutz’s favorite “hot core” trap loads used Green Dot, one grade slower than Red Dot (but not as slow as Unique). But if Green Dot was good, why wasn’t Unique better? Many of the major shell companies use fast powder in their handicap loads and the patterns are very good. The jury seems to be out on this one. It’s for sure that powder is only one of the components in the shell that determine pattern. One thing that’s indisputable is that fast powders are cheaper to use than slow powders because you use less fast powder. The shell makers are very aware of that.

I’ve been satisfied with the patterns from my fast powder loads so I’ve never bothered to do the pattern board work. It wouldn’t take long. Get a pound of Red Dot and Unique or Clays and Universal, make up equivalent loads and have at it. After five shots of each, you’ll find out which one your gun likes. Then start changing wads and shot if you want some real differences. Crimp depth can matter too. And hull brand. And your chokes- they can really make differences when you switch around.

3) This one I’m pretty sure of. Fast or slow powder won’t make a significant difference in your muzzle velocity. Either powder gets the vast majority of its burning done well before the gas ports of an auto.

There it is. Now you know more than I do. I’m going shooting.

Best regards,

Bruce Buck
The Technoid writing for Shotgun Report, LLC
(Often in error. Never in doubt.)

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