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Benchmark’s two-groove .308″ Barrel

Benchmark’s two-groove .308” barrel

The History

Rifle barrels with only two grooves are nothing new. As near as I can tell the Lee-Enfield was the first to employ two groove barrels (at least for high power rifle cartridges).  They apparently conducted a test in May, of 1941, where they fired 500 rounds through a five groove and a two groove, accuracy testing with the barrel hot, and checking for fouling and barrel wear. The conclusion was that there was no significant difference in accuracy, fouling, or wear. The reason they went to the two groove was an apparent advantage in the ease and speed of manufacturing barrels with fewer lands and grooves.  Remington later conducted some of its own testing with favorable results, showing accuracy equivalency at closer ranges and if anything. superior accuracy for the two-groove at longer range (see references). The Springfield 03-A3 apparently followed suit in using two groove barrels for the same reasons.
When rifling is made by pushing or pulling a button there is no cost or speed of manufacturing advantage with one rifling layout over another. In rim fire competition, there is believed to be an advantage in two groove barrels with respect to wind drift. Benchmark Barrels has been making two groove rim fire barrels for many years, and they’ve had great success, setting more than a few world records along the way. Ron Sinnema (owner of Benchmark) decided he wanted to try a two groove barrel for center fire rifles. So, he ordered  a two groove button for .308” 1:10 twist. He thought there may be possible advantages in wind drift, fouling, or barrel wear. There also was a chance that it would not shoot well with only two grooves. The accuracy standards of aftermarket barrel manufacturers today are much more stringent than what the old Lee-Enfield military rifles were held to,. The fact that two groove barrels seemed to match the accuracy of the five groove barrels in British military tests is almost irrelevant.

 

The Build

The 2011 Oregon Sniper Challenge had been caliber restricted to 308 win, and 223 Rem. Ron and I had participated the previous year, but neither of us had 308’s (I‘ve had more than a few, but none at the time). I did have a 30 cal. 3 groove 1:11 twist Benchmark barrel I’d purchased a year before for a 300 win mag project that never came to fruition. I also had a Mark X commercial mauser action, a cracked CZ 750 stock that I planned to repair and re-inlet for the mauser action. I had a custom built detachable box magazine system made by Randy Ketchum (owner of Lynnwood Gun) that used Accuracy International long action magazines. Ron decided to pass on attempting to sign up for the match this year. I decided I would have the above components made into a 308 win. When I told Ron my plans he remembered that he’d made a two-groove barrel, and had yet to test one and see how it would work. It was then decided that my “odd duck” mauser project be used to test the two groove benchmark barrel.  Coincidentally, all this happened 70 years after the Enfield test… to the month! This particular build was definitely not an ideal test platform, but with a trued action, good bedding, and a precise chamber job, we had no reason not to expect solid accuracy.
The actual build went faster than I could have expected. The barrel came out to a handy 23 inches. I went with a reasonably heavy custom contour we affectionately refer to as the “Jack Ass Contour”. It was designed by a buddy  for long range “practical“ matches (we told him he should name it… he was joking, but the name stuck).  It’s 1.250” for the first 5” it tapers to .900” at 14”  and from there, it tapers to .780 at 30” (AKA: Stuurmans’ modified palma). The stock was repaired, the custom DBM was working great. I expected an accurate, functional, match rifle. I also expected the rifle to be a little schizophrenic in nature due to the hodge-podge of parts I collected for the build. This was not the case. The parts worked together almost flawlessly. The CZ 750 stock looks as if it was always meant for that action. It is an excellent design, very similar to the AICS which I’m very familiar with. For straight up ergonomics the CZ stock is better in some ways. The thumbhole is slightly more comfortable and placed farther from the trigger (one of the few issues I have with the AICS), the drop near the magazine well is deeper, and I prefer the  feeling of textured composites over the plastic feel of the AICS. The cheek piece design and butt-pad are inferior to the AICS but I modified the butt-pad. Of course the AICS is much tougher, but the CZ stock is much lighter. The AICS mags used in my custom DBM stick just below the mag well, I’d rather be flush, but it works out very well and is easy to install and remove magazines. There are stops built in, so the magazine cannot rub against the bolt. The trigger sits farther back than would be ideal so I planned to modify the trigger shoe, but never got around to it. I was accustomed to it quickly anyway.
 
Field Testing

As soon as I got the rifle in my hands I got the  load testing under way. I planned to use 155gr SMK Palma bullets and shoot them at a good clip (over 2800fps) with Winchester cases full of Varget, and CCI BR-2’s. I couldn’t  find any 155 SMK’s locally but I did find Berger 155 VLD’s. I couldn’t (or wouldn’t) wait, so I started load development. I always run “Optimal Charge Weight” loads. The high OCW for this combo came out to 47.2gr of Varget. This netted me 2840fps in the 23” barrel.  Accuracy testing put out groups between 1.25” to 1.6” at 300yds for 5 shots (prone off bi-pod).  I was getting better accuracy with 175gr SMK’s over 42.5gr of Varget, but I was after the better ballistics offered by the light sleek 155gr bullets. I decided to try out a few 155gr Lapua Scenars I had collecting dust in the reloading room. With the same load of 47.2gr of Varget and the BR-2 primers accuracy improved. The rifle was now grouping between .8” and 1.25” in good conditions at 300yds (also prone/bi-pod). I was very pleased with these results. I double checked that I was still in the OCW and it was. I felt confident this load would shoot well in a wide variety of temperatures and conditions. The more we took this rifle out for long range testing the more confidence grew. It became obvious fairly quickly the rifle was excellent, the slight stress associated with “testing” wore away and “two-groove” became the punch line. “I told you two-grooves shoot great from improvised rests”…. “You probably don’t have to hold for wind at all with the… two-groove”…. “Heck no, I’m not going to clean it, it’s a two-groove. Remember?”
Not long after finding the above load I took the rifle to a long range match. The Big Wheels Steel Challenge would give me an opportunity to run the test rifle with “experimental” barrel out to 1300yds! Before the match I checked my drops out to 1330yds with Ron Sinnema and Chris Marti. Ron brought his 243 shooting the 115 DTAC’s and Chris brought a 7wsm shooting 168gr Berger VLD’s. There’s no arguing the superior wind drift of the 243 and the 7wsm, but we were all surprised by the 308’s accuracy at such distance. My particular load would be fully sub-sonic by the time it reached the rock we were shooting at. Even still the 308 was easily the most accurate of the three. We didn’t shoot at paper so I will not speculate on exact group size, but I can honestly say, I’ve never witnessed a 308 hold that kind of vertical dispersion beyond 1100yds. I’m sure some have achieved similar results with 210gr Berger VLD’s or 208gr A-max, but it was more than surprising from a 155gr bullet. At the Match (BSC) the rifle performed flawlessly all the way through. I managed to take 3rd place, and was the highest scoring 308 by a long ways. I managed to clean more than a few stages.  The longest stage (1250yds and 1300yds) were dropped from the score but I did hit both targets (neither with first round). I don’t believe dropping the score on that stage would help or hurt enough to make a difference although I did out-score the match winner on that stage, a buddy who was shooting 115gr DTAC’s at just over 2900fps (also a Benchmark Barrel). Then came the Oregon Sniper Challenge. Things didn’t go so well at this match. The rifle shot great all the way though but my score on the first stage took the wind out of my sails to say the least. Although shooting a perfect score (200pts) shooter and spotter switched scores so I was awarded only 60pts. This would be fine had we suffered a communication issue (what they were apparently attempting to score), but unfortunately the shooter I was partnered with didn’t have a proper zero. The highlight of OSC for the rifle and I was the long range stage (800yds). We had to shoot 10 shots with no sighters at a silhouette. I had the 3rd highest score on the stage 85pts out of 100, the highest score was 89pts. I could have scored better but I  hedged my bets by shooting the last 3 shots held progressively farther into the wind than what I felt was correct. If I’d have been wrong with my original wind call, it would have salvaged the stage and allowed me a few points where I may not have scored otherwise.

 

Breaking It Down

By now I have over 750 rounds through this two-groove 1:10 twist Benchmark Barrel. Does it work? Absolutely! I have been very impressed with this rifle from the beginning. So here’s my final analysis on the “two-groove”…..

 

-Accuracy
with two grooves: This rifle is very accurate. I would not expect any improvement in accuracy at any range with a more traditional 3-groove, 5-groove or what have you. I shot one group at just over 650yds that was just under 2.6” for 5 shots.  Had I shot more that one group at that distance, I’m sure some would be better and some would be worse, but there’s no doubt the rifle consistently displays excellent accuracy. I’m still curious if the unusual long range subsonic accuracy I observed with this rifle could be attributed to the two-groove barrel.
-Wind drift
with two grooves: Unfortunately I have not seen any significant advantage in wind drift with the two-groove layout. The bullet drifts about what the drop chart says it will. I would guess that when a bullet is super-sonic the grooves cut into the bullet don’t  effect bullet flight much, if at all. If one was to test this design subsonic, it may display the same advantages many observe in rim-fire.
-Fouling
with two grooves: This barrel has always been very easy to clean. Copper fouling is on par with other hand-lapped Benchmark’s  I’ve owned (very little copper fouling). I will say that powder fouling does seem to be better with this design. When I entered the BSC match I already had over 200 rounds through it without cleaning. Looking down the barrel with the naked eye you’d never know it.
-Barrel Wear
with two grooves: Had we chambered the barrel in 300 Ultra Mag, we could have assessed barrel wear. But a 308 win is very easy on barrels as it is. I would not be supprised if barrel wear would be reduced by having less lands and grooves but I have no way of knowing conclusively either way at this time.

 

The ultimate question is, would I use a two-groove instead of three or five grooves. I honestly would. As did the British when testing the two-groove, I see no down side. They seem to powder foul less, the barrel may last longer, there is no degradation of accuracy, there may be advantages when shooting sub-sonic. So without a doubt I would use a two-groove again. The 1:10 twist is a little faster than I would choose for 155gr bullets but it worked perfectly.  I think the 1:10 two-groove would be ideal for a custom 308’s and 30-06’s used for shooting heavy bullets over 200gr. I believe that would be the niche for this particular barrel (although with my results I’d run it in a 308 w/155’s too!) Ron believes the two-groove may be a little harder on bullet jackets so a 300 ultra mag shooting bullets at high MV’s may be better served by a 5r style rifling. I’ve always liked to use something a little different… You probably realized this when I described the custom Mauser that would become the test bed of the two-groove. I have a feeling though, that a few years from now the two-groove may not be nearly as “different” as it is now. I look forward to seeing the results of others with this design.

 

I want to Thank Ron Sinnema and the rest of the Benchmark crew for making this build possible. I also want to thank Randy Ketchum for an excellent custom DBM. Thanks to Jeff Gay and Steve Huisman for putting on some great matches. I had allot of fun with this rifle and will continue to enjoy it in the future.

 

By Lucas Beitner

 

References:
The Lee-Enfield Rifle”, Major E.G.B Reynolds
http://www.remingtonsociety.com/rsa/journals/two-groove

Discussion

  • Mario Bernheim

    hi! what COAL did you seat with 175SMK?