doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com ammo pressure question

Picking up a box of .410 ammo at Wally World today, I noticed several boxes Winchester AA in 20 ga marked "2 1/2 dram" and "7/8 oz" shot. This is how my RST ammo for my 20 ga Flues is marked. Would the pressures here be similiar enough not to damage my old sxs? Was not sure if there was another variable I am missing in the pressure equation....

Dave

Dave,

Unfortunately there is NO way to determine pressure from the combination of dram equivalence and shot load. Cartidges with lower dram equivalence "tend" to be lower pressure, but you cannot determine that for sure simply from reading to shell box.

Dram equivalance is an archaic term now used to give an indication velocity. However, you have no way of knowing how the manufacturer is achieving that velocity - i.e, the specific powder the manufacturer is using, and what that powder's pressure curve looks like.

Often with promo "lite targte/field" loads, the maufacturer will utilize relatively small amount of fast burning power (to achieve cost savings) but these loads still bump up against the SAAMI-maximums in regard to pressure.

Lower pressure loads are readily availble via mail order so why risk pounding you old gun to save yourself $20/flat.

Good luck,
Ken

Ken-
I knew there had to be more to it, I will stick with my RST's and keep the old girl safe....

Thank you,
Dave

Dave:

SAAMI standard is SAAMI standard. American manufacturers must merely load ammo that equals or is less than a stated Maximum Average. Case length does not matter ( except for 12g 3-1/2" and .410 bore) .

The current SAAMI standard is ANSI/SAAMI Z299.2-1992.

.410 bore 2-1/2" has a Maximum Average Pressure of 12,500 PSI
.410 bore 3" has Maximum Average Pressure of 13,500 PSI.

As Ken points out, there are 2 basic ways to achieve a particular velocity in a shell with a given shot charge.

1) small amounts of fast burning powder, which produce the stated velocity cheaply, but by developing high pressures.

2) Larger amounts of slower burning powders, which work at lower pressure, but cost more, sometimes much more.

If you found them at Wal-Mart, in the "el cheapo" section , it is like they got cheap by using the fast burning powders.

Stick with the good stuff.

My ideas - do what you want.

Regards

GKT

Dave,

You have two very knowledgeable responses.

There is also one more element. Most shotgun users are not shooting a sxs. Many are shooting a semi-auto. The ammunition makers do not want angry calls about their ammunition not cycling the customers semi-auto. So, they make sure there is enough pressure to do the job. The reason that true low pressure loads are more expensive is the market is much smaller.

Pete

Excellent, well thought out repsonse from very knowledgeable people. I appreciate the info gentlemen, thank you!

Dave

As regards use in semi-autos, most modern SA shotguns are gas operated. Now if one studies the pressure curves of equivelent loads using different powders it will generally be noted the "Low Pressure" loads actually have a higher pressure down where the gas port is. I think what comes into play mostly is at the "Low End" of the shot charge wt. When I began shooting a shotgun back in the '50s min factory loads in 12, 16 & 20 were 3-1; 2½-1 & 2¼-7/8 respectivly. Recoil operated autos when set for light loads would operate these guns reliably. With the introduction & proliferation of Gas guns these loads in 12 & 20 were increased to 3¼-1 & 2½-7/8 with the 16 remaining unchanged, Why?? No-One was building gas 16s!! "My Theory" is this increase in Dram Equiv was made in order to stay with a fast powder which was running out of steam prior to passing the gas port. No doubt the same ballistics could have been maintained by a switch to a slower powder which would have maintained adequate pressure at the port, but this would have required a larger increase in the wt of powder charge than a simple increase in the fast powder charge. Likely these older loads were operating enough below SAAMI max the increase could be made. These older loads were incidently much more comfortable shooting in light wt field guns than their modern equivelents, the price we double gun lovers have had to "Pay" for the "Reduced Recoil" of the gas guns.
Bottom line is it was all "Economics", not that a load using slower powders could not be made to operate a gas gun.

Miller,
Where can I find the pressure curves you refer to?

Chuck;
An old set of curves published by DuPont has been posted here on the BBS several times. Admittedly they are of obsolete powders, but show the general characteristics of a shotgun pressure curve. Those curves were run with Black, DuPont Bulk Smokeless, DuPont MX & DuPOnt Oval. All were loaded to a 3 dram or equiv charge with 1¼oz shot. MX showed the highest pressure, then bulk, then 3FG Black & finally Oval (a dense progressive powder intended for heavy charges at high velocity, actually an underload in this case) with the lowest. All pressure curves crossed at approx the end of chamber with the Oval giving the highest out to 10" which was as far as the chart was carried. As by this point the curves had pretty well smoothed out presumably they would retain the same relative position on out to the end of bbl.
It is really rather logical that if two powder charges give the same wt of shot, the same MV but at different max pressures, the low pressure one has to play "Catch-Up" down some portion of the bbl. If the same work is done, then average pressure over the length of the bbl is going to be Essentially equal.
Given the position of the gas port it would seem certain the load with Oval would have operated a gas gun rather more "Smartly" than would the MX load even though the MX had a "Higher Pressure", but remember we are speaking of chamber pressure here. It simply seems that many simply cannot look beyond "Max Chamber Pressure" & base all characteristics of a load solely on that one data point.

Do I hear my cue?



Pete