Something interesting to chew on.

*This is an interesting one time test I thought you would find interesting. A good friend sent it to me to mull over. It supports the efficiency of many of the GNR cartridges design characteristics. My own thoughts is how does recoil fall into this? Backthrust surely would play a role?! Gary has made some 9.3X74R barrels and says they are mean and nasty compared to his .350 GNR, .366, and .378 GNR. The 9.3x74R is very tapered....just saying.

NO Arguing just some fun food for thought. If you REALLY want to know the folks who thought this up you can PM me.

(Cut and pasted from Email) - SB

Just my two cents worth on BACKTHRUST......

The Test: Since I do a lot of rechamberings on the T/C Contenders, this is what involved the main area of testing. I took several barrels that I had, to the range where a friend and I had built a "stand" to hold a special vice and fixture that we had made specifically for the testing of these Contender barrels. The barrels were locked into this fixture WITHOUT the use of a frame. A "firing device" that was attached to each barrel, via the rear mounting holes in place of a scope base.

The first test involved firing the 30-30 Ackley Improved. With no case head support what so ever, a round was fired and it was found that there was "0" rearward movement of the case! All loads tested were full power hunting and target loads, with the chamber wiped out with alcohol and dried, before the next round was fired. Everything had to be exact, round after round. The test was repeated five times for each cartridge listed. All 5 30-30 Improved cartridges showed "0" backwards movement....

The next test involved the 250 Savage Ackley Improved. Again, all 5 cartridges showed "0" rearward movement when they were fired and two of the cases were actually .002 "deeper" in the chamber after they were fired than before. Reason being, the strike of the firing mechanism had sufficient enough force to drive them further into the chamber....

Next in line was the 7mm T/CU. Same thing as the rest. NO REARWARD MOVEMENT - meaning, absolute ZERO BACKTHRUST!!!

Next came the 35 Remington. well, this was a horse of a different color. This case shot out of the chamber end so fast, that it removed my "firing mechanism" with it! I went back and made another one, with slight modifications. I fired off the other four rounds and they all "shot" out the chamber end as well. I set up a piece of pine 2X12 three feet behind the chamber when I went to fire off the last round and that case blew right through it!!!

The next round to be fired was the good ole 30-30 Winchester. It did exactly the same as the 35 Remington! I decided to set my chronograph up behind the chamber to see if I could get a reading on these "projectiles" coming out of the chamber end. I did, it did and I clocked that sucker moving out at 1900 fps!!!

Next was the standard 250 Savage. This was the only one I tried factory ammo in. All of the others were handloads that were developed for those barrels and all of them had the bullets out to touch the rifling. I did shoot two handloads in the 250 Savage as well but all tests were the same. All of the 250 Savage loads exited the "chamber end" at well in excess of 2,000 fps....

I have found that it all boils down to this: if you have a clean and dry chamber and an "Improved" case, meaning "minimum body taper", that when the round is fired, you transmit next to zero or zero backthrust against the frame or bolt! Cases that have a "taper", will always transmit a heavy backthrust against the frame or bolt! How much backthrust it transmits will depend on the taper of the case. Naturally, the greater the taper, the greater the pounding....

Like I said, this is just my two cents worth. I learn something new every day and learned alot in performing these tests. It has helped me alot in the design and development of my own wildcat cartridges....


P.S.
I apologize for not mentioning the primer! I had my notes with me beside the computer and somehow over looked them.

Of cases that had been fireformed and fired one other time, the primer had backed out of the "improved" cases anywhere from .012 to .080 of an inch! Cases that had been loaded half a dozen times or so and the primer pockets not quite as snug, probably 90% of them blew completely out of the case! With a piece of 3/8 inch ply wood set up a foot behind the barrel, half of the primers blew completely through it. A lot of them were protruding out the back side and were easy to pull out with bare fingers....

We thought of this before we ever fired the first round. Getting hit by one of these "primer" bullets, would be like being shot with a 22 rifle!!!

I have checked the lug thickness on different rifles that I have built for different customers, namely the ones that shoot a lot, and it is amazing to note the lug "compression" on say a 300 H&H vs. even a much larger round like the 30-378. Even with the larger case head diameter and much higher pressures of the 30-378 case, the 300 H&H was still harder on the locking lugs than the much larger Weatherby case....

Case taper most defiantly plays a huge role where "backthrust" is concerned....