In the early spring of 2008, we started looking for a way to reduce peak chamber pressure to allow more and or different powders to be utilized in any given round for additional velocity. The primary concern in this project was, of course, accuracy retention.
We knew that we could gain 100 fps by simply adding a gross amount of “freebore” thereby reducing the peak chamber pressure and allowing the addition of more powder to come back to max pressure. This technique was well explored by Roy Weatherby of course. The downside of this is that initial accuracy and long term accuracy were affected in this process, and it was not the substantial velocity increase we were looking for.
Of the bucket full of ideas we kicked around, the one we kept coming back to was the current version of the +P we now use on the 338 Edge +P. This was an idea of a stepped throat, instead of the bullet having to initiate twist and take the engraving of the full depth of rifling all at the same time we thought that removing a portion of the rifling for a given distance would allow the spin initiation and initial engraving process with much less pressure before coming into full rifling contact. It was my opinion that this would reduce peak chamber pressure enough to make possible the gains in velocity we were looking for while retaining accuracy. I even in the back of my head had the idea that this reduced portion of rifling would allow the bullet to align better with the full rifling portion of the bore and give a slight accuracy increase. It was with these ideas we started testing.
The first thing was to select a cartridge to test this out on. I have more experience with the 338 Edge than all the rest put together so I thought this base of info would help in testing. I put together a solid bottom Remington 700 long action and chambered the barrel in standard 338 edge, installed a break, etc. I shot the base rifle in standard chambering design and it performed like all of the rest. Standard group sizes were in the .240-.290” range with our standard test load of 90.0 gr H1000 and the 300 gr SMK, all pretty normal stuff. The next step was to “+P” this same rifle, and I did (before you ask, I’m not telling how much reduction or for what length I machined the chamber several barrels died to give up this info). Some polishing of the machining was required and turned out not to be as simple as I expected. With the first “+P” chamber cut, it was testing time.
I loaded our standard 90.0 gr H1000, 300 SMK load again, and fired it over the chronograph. I blinked and checked the chronograph to make sure everything was working and set correctly; it read 2689 fps. Now I had expected it to reduce chamber pressure and velocity somewhat, but I didn’t see that one coming. I started loading in 1-grain increments until I started seeing pressure signs. When I fired the 97.0-grain load, if you held the case just right in the light, you could see the faintest ejector marks. I loaded 98.0 grains, and the ejector marks were now visible but were not wiping brass, the bolt lift was two finger easy, extraction was a piece of cake, and the web showed no measurable expansion. So in the interest of science, I fired a 99.0 gr load. This was clearly not what the brass or rifle had in mind. The ejector mark was a wipe, the bolt lift resisted, and “clicked” at the top of the bolt lift. This is, of course, way too much pressure for anything. I backed the load down to 97.0 grains and shot some groups for accuracy, velocity, ES etc. The average velocity was 2931 fps, the ES was 8 fps and the accuracy was .200”-.260”. I saw this as a glowing success, but you have to wonder if a little is good, is more better? Well, no, it wasn’t as it turned out after extensive testing of different depth/length combos our initial settings seemed to be about the best balance. It was at this time I started the patent process, learned a lot about that I wish I had known earlier.
While I had other tests in mind, the major issue in my mind was, will this modification affect barrel life? I wanted to solve this question and solve it fast. I shot the rifle quite a bit but got bored of 100 yards shooting and was pretty much limited to it this early in the year. So I enlisted the help of a good friend. It went like this, “ Sam stops by the shop. I have something for you to check out”. When Sam showed up I loaded the rifle, a couple of boxes of 500 ct SMK’s, an 8 pounder of H1000, and told him to keep detailed notes about overall throat wear, velocities, and accuracy while he was shooting up this pile of bullets and powder. Oh, yea, and could you get it done pretty soon? He looked at me like there was something wrong with me, smiled, and left. When Sam finished that portion of testing, it was easy to see the “+P” was going to have very similar barrel life to the standard Edge depending on how good of care you took with it (I still have the barrel 1150+ rounds still shoots sub .5 moa).
I settled on the specs for the 338 Edge +P reamer and had Dave Kiff grind a couple of carbide reamers with the +P specs incorporated in them. It was about this time that the final development of the LRKM rifle was coming together, and they seemed like a match made in shooter heaven. The very first prototype LRKM was chambered in 338 Edge +P, and I am still shooting it today. With its’ 30” Hart barrel it ran 2923 fps using 97.0 grains H1000. I started looking at some other powders that would give more velocity, single-digit ES, and very low-temperature sensitivity. These requirements really only left one powder worth looking at (although we tested several), VV N570. I started loading the 570 up with SMK’s and reached 3040 fps with 101.0 gr. I shot more but backed down to this load as a good safe stable load for a number of reasons. Well, with the velocity so high I figured the ES would be horrible, but was pleasantly surprised when it turned in an ES of 11 fps. Well, now, this is just looking too good, and I am waiting for the other shoe to drop when I tested temperature sensitivity. I froze a group of rounds to 22 degrees and warmed up some to 104 degrees. Again the ES was 9 and 12 fps for each group, and the average velocity difference was three fps. (Note: Small celebration was in order here) It was not too long after this testing that we received our final patent paperwork and processing to make it officially ours.
So as of this writing, the “+P” design in 338 Edge has gained a little over 240 fps through reduced chamber pressure and allowing different powders. It has actually increased accuracy and has not affected barrel life in a negative way at all. The only thing to do next is test the design out on other calibers. I am thinking a 7 WSM with 180 Berger’s would be a good candidate. I am not saying we have started testing already, but I am pretty sure the 7 WSM +P will drive them over 3000 fps out of a 26” barrel. Stay tuned for more +P developments in the near future.