Does Bearing Surface (Friction) matter?

I wasn’t sure what to call this article, so for now let’s call it…

Is it weight or Bearing Surface that counts?

This started quite innocently on my last trip to the range. as I had intended to work on some loads for the 30-06 and 300 Win Magnum using Hornady 180 gr InterBond bullets.

In working up the loads a grain at a time, I noticed that in both calibers and with all powders; I had to stop 1 or 2 grains short of where I would have anticipated the maximum.

An example to illustrate would be the 30-06 where normally I use 50 gr of AA 4064 and 180 Hornady RN bullets for 2620 fps. Now with 48 gr of the same powder, but with the 180 InterBond bullets, I get 2688 fps and if I go to the 50 gr load, I got cratered primers and 2786 fps out of 22” barrel.

In thinking this thru, I figured it must be the bearing surface that makes difference.

Not to bore you with a whole lot of numbers, I’ll limit it to the following 30 cal:

180 gr Hornady InterBond is 1.372 long 180 gr Hornady SPBT 1.276

200 gr Speer Hot Core 1.259 220 gr Nosler Partition 1.362

180 gr Hornady SST 1.346 180 gr Swift Scirocco 1.423

180 gr Barnes TSX 1.383 (with 4 grooves)

180 gr Barnes MRX 1.364 (has Tungsten core)

On the next trip to the range, I was armed with many more bullets to experiment with. I used the same AA 4064 loads I tested the 180 Scirocco, TSX, MRX and SPBT for comparison.

In each case, anytime I exceeded the 48 gr load, I got excessive pressure signs. Contrary, with the orginal 180 gr Hornady SPBT, even at 50 gr, there was no problem.

What I was able to figure out is the Scirocco and InterBond are so long that they even exceed the 220 Nosler Partition. This long bearing surface has got to make a difference.

I also tried some loads in the 300 Win Magnum as I wanted to compare the 220 Partition to other 180 gr bonded core bullets.

My favorite load is 69 gr of Rel 22 and the 220 Partition, so I loaded up some 180 InterBond and Scirocco bullets and found that we were pushing the limit with the 69 gr of powder. This is a strange situation, but remember, the 220 Partition is actually shorter than the 180 InterBond or Scirocco. Could it be that the bearing surface, and consequently the added friction because of the bearing surface, play a part in this?

I then tried another favorite load which is 71 gr of H 4831 SC. The results are as follows:

180 InterBond = 3026 180 SP BT = 2881

180 TSX = 2911 180 Scirocco = 3055

180 SST = 2853

Obviously again, the bearing surface must have a lot to do with the increases velocity.

This was so much fun with the 30 caliber bullets that I just had to try it with the 338 Winchester Magnum too.

First I measured a group of bullets and the results are as follows:

225 gr Speer SP BT 1.295 210 gr Scirocco 1.391

210 gr Barnes TSX 1.352 225 gr InterBond 1.422

250 Hornady RN 1.259

250 Rhino 1.416 (with 4 grooves like a TSX)

As you can see here too, the 225 InterBond is longer than the 250 RN.

I loaded up all of these with my favorite load of 73 gr of Rel 22 and again the results were quite interesting.

225 InterBond = 2890 fps 250 RN = 2680

210 Scirocco = 2833 250 Rhino = 2711

210 TSX = 2755 225 Speer = 2780

As you can see, that 225 InterBond is really moving. Unfortunately I did not have any 225 gr. Scirocco to compare it to, although the 210 gr Scirocco is close behind.

Again I can only feel that the heavy jacket and long bearing surface of the InterBond and Scirocco bullets has to play a part in higher pressure and higher velocity. That really long 250 Rhino could, or should, have produced more velocity, but maybe it did not build as high a pressure because of the 4 relief grooves in it like the TSX.

The long and short moral of the story is this. Treat all of these long bullets like they are much heavier than they really are. Don’t just take standard 180 gr data and load them. Start way under and work up, or look up 200 gr data, and start there, but start low.

Feel free to write or call Terry anytime at tblauwkamp@superior-sals.com