Low Pressure, Low Recoil?


Dear Bruce,

How much does chamber pressure affect recoil if at all, I have noticed in my reloading manual that the chamber pressure for my standard load of 700x with 18.0 grains has about 10800 psi and the same load with the same components with the exception to changing powder to 800x would be 7200 psi for the same 1200 fps load.

Could the amount to 30%+ less recoil, or is it the difference between powders that push compared to a sharp rap.

Thanks,

David

Dear David,

It is interesting to note that the formula for free recoil which SAAMI uses could care less about chamber pressure. It does care about the number of grains of powder used. Powder weight is added to the ejecta weight. What this means is that “slow” powders that slowly push rather than sharply poke generally take a few more grains (Red Dot vs Unique or 700X vs 800X for example) to get the same speed. According to the formula, more grains equals more recoil, so the slow powders technically kick very slightly MORE, not less, than fast ones.

In one load combo, 1200 fps is obtained with either 17.5 gr 700X or 22 gr of 800X. The calculated free recoil with the 700X is 17.49 foot lbs, but with the 800X it is 17.92. Not much of a difference, but a difference to be sure. Mathematically, slow powders kick MORE than fast ones. Chamber pressure for the 700X load was a high 11,300 psi, while it was a low 7,800 psi for the 800X. Chamber pressures do not affect free recoil calculations.

Of course, free recoil and the subjective recoil that you feel can be two very different things. I have run some blind tests with a group of shooters comparing “slow” and “fast” powders (Red Dot and Unique). The shells were reloaded to the same velocity. The slower powder generated about 2,000 LUP less chamber pressure according to the manual.The end result was that in a blind study opinions of the six shooters were evenly divided as to which kicked more. Half said the slow powder did, half said that the fast powder did. In short, no noticeable difference.

We do know from the study of gas guns that stretching the recoil pulse out over a longer period of time reduces perceived recoil for most people. The recoil curve of a gas gun looks a bit like a Bactrian camel with two low humps, while that of a fixed breach gun looks like a dromedary camel with one sharp spiky hump. The areas under the recoil curve are the same, but the shape of the curve differs because the gas gun stretches the recoil out over a longer period.

It would seem that slower burning powders would also do this. They burn slowly up the barrel so they are, in effect, burning in a larger area, thus generating less pressure in any one spot and taking a bit longer to do it. The analysis is accurate, but the problem is that the practical difference in time of burn is so slight that the human body can’t detect it for the most part. It is like porting a shotgun. Yes, technically it works, but it doesn’t work enough to notice.

Bottom line, lower chamber pressures do not mean lower calculated free recoil. They may mean lower felt recoil, but probably not enough to matter.

By the by, I DON’T recommend that you use 800-X in a 1-1/8 oz target load regardless of what the manual says. Yes, it’s safe, but you aren’t going to like the muzzle blast. 800-X works fine with heavy field loads of 1-1/4 oz and up, but for target stick with 700-X. Trust the Technoid on this one.

Best regards,

Bruce Buck
Shotgun Report’s Technoid

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