doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com burn rate and psi

I would appreciate hearing your opinions on the importance of burn rate on low pressure loads. I feel the need to know more about its impact on shooting vintage guns. Thanks,Justin

Just my take on this subject & I know in advance many will dis-agree. Burn rate of shotgun propellents are controlled primarily by two means; Configuration of its granules &/or chemical coatings which "Retard" its burning.
Slow or so-called "Progressive" powders were "Not" originally designed for the purpose of producing "Low" pressures, but for pushing larger shot wts to normal or higher velocities without exceeding the max allowable pressure range.
Faster powders will therefore burn more reliably under low pressures & under a greater variation of temperatures than will slower powders. Most of the vintage guns with which we deal, will, if in sound condition quite readily handle loads producing pressures in the 7-8K psi range & were actually designed for loads up to about 9K psi. My personal preference is to use the lighter shot charge wts for the gauge, keep my velocities no higher than 1200 fps, usually around 1100-1150fps & to then use the fastest powder of my preferred brand which will give pressures in the 7,000-7,500 psi range. A gun which I don't figure capable of hadling this I don't shoot.
"IF" I were loading for very high volume shooting in warmer weather "Only" I "Might" use a load which dropped the pressures to about 6K, but no lower.
You of course realize if you push a given wt of shot to a given velocity, you do not really "Drop" overall pressure, only re-distribute it. "IF" you push for instance 1oz of shot to 1200fps with two loads, one at 5K & the other at 7K, the 7K load will obviously have the highest "max chamber pressure". Not so obvious, the 5K load will have a "Higher Pressure" for the rest of the bbl. It is worth noting I think that bbl walls are "Always" thicker over the chambers than elsewhere, this was for a reason. Many will look at those 5K & 7K figures & think they are getting something for nothing, but "There Ain't No Free Lunch". What you "Save" at one point you "Pay For" at another.

Well said, 2P. I agree 100%. I use Nitro 100, one of the fastest burning powders of the bunch, in my 12 ga. loads. With light shot charges(7/8 or 1 oz.) it produces relatively low pressures. It is also economical to load, due to the small quantities required. I load it for about 1150 fps. Recoil is a bit less, as well, compared to that with powders requiring several more grains, to produce the same velocity.

Miller, I am more concerned with the overall mechanics of the gun,not the integrity of the barrels. When a fast powder is used and the pressure is expended within the chamber does this produce more of a jolt to the gun. I'm thinking of the rib welds and the joint of the gun. Justin

well, Justin, you have to order crusher test pressure of your loads.
Actually there is no straight dependence between powder burning rate and load impact on gun.

The recoil you or the gun receives will be dependent on the load and velocity. Neither of you will notice the difference between slow and fast burning powders, unless the slow powder requires considerably more powder, to produce the same velocity. Then there will be more recoil.

My Sherman Bell articles are currently out on loan. However, I recall that in at least some of his "finding out for myself" tests, he placed strain gauges at several points along the barrel. I don't recall the phenomenon of higher pressure farther down the barrel with loads that generate lower chamber pressure, but I could well be misremembering.

2-piper, let's say you have two 7/8 oz. 12-ga. loads at 1150 fps, one generates 4300 psi chamber pressure, the other 7000 psi chamber pressure. Are you saying that the 4300 psi load results in a higher pressure midway down the barrel than the 7000 psi load?

The issue that was brought up with 2Piper has to do with the concept of “work” on the projectile. In order to have the same velocity, the work done in both cases must be the same. Mechanical work is the amount of energy transferred by a force acting through a distance so in this instance it is the pressure force over the distance down the barrel.
A fast burning powder has a sharper pressure build closer to the chamber but decreases more quickly than a slower burning powder. If you were to plot the pressure profile down the barrel, the area under the curve represents the work done to the load. If the velocities are the same, then the work is the same. Given that, the 7000 psi load will have a rapid rise and decrease in pressure whereas the lower pressure load will maintain the pressure longer.
At some point in the barrel, the pressure curve will drop below the 4300 psi curve. So although the peak pressure at the chamber is higher with one load, the other will have a higher barrel pressure for some portion of the time.
z

Zwego has worded this far better than I am capable of. The bottom line though is plain & simple if one stops & applis a little Thinking. If the two loads start with a different pressure level the one starting lowest has to gain it back at some point or it would not produce the same velocity. So "YES" if the teo loads produce the same velocity to the same shot wt & one has a peak of 4300psi & the other 7000psi, the 4300 psi load will have higher pressure down the bbl. It will in fact most likely have higher pressure "EVERYWHERE" except over about the first 3" from the breech. The higher pressure area of the 4300psi load will be spread out over a much longer length, so it will never exceed the other by 2700psi.
Pressure is of course not the sole factor involved here so it would not be save to say the "Average" pressure over the entire bbl length would be identical, but pressure is the major factor & that won't be far off.