October 6th, 2010

6mm 90s & 95s — Lighter Bullets May Be Better for Mid-Range

Many shooters using the 6mmBR case or a 6BR Improved (6 BRX, Dasher), automatically assume they should be shooting the heavier 105-108gr bullet designs because these offer the “best” ballistic coefficient attainable with a bullet that can work in an 8-twist barrel.

95 grain Sierra MatchKingHowever, if you are shooting a 6BR at medium ranges, say 250 to 400 yards, you should seriously consider trying the 90-95 grain class of bullets, which includes the Berger 90gr Match Target BT, the Lapua 90gr Scenar, the Berger 95gr Match Target VLD, and the Sierra 95gr MatchKing.

First, you may find that, in your barrel, the 90-95 grainers are easier to tune in terms of seating depth, and they may offer somewhat better raw accuracy — yielding smaller groups than the heavier bullets. But remember — each gun/barrel is different.

Second, another advantage of the 90-95s is that you can fill the case fuller with the Varget/RL-15 class of powders (with appropriate throats). You can use more powder and therefore get closer to an optimal 100% case fill. With a 95gr VLD seated long we were able to get virtually 100% fill with a slow lot of Varget. Don’t try that with your 105s!

Lighter Bullets Offer More Speed in a 6BR
You’ll find that, in a standard 6mmBR rifle, you can drive the 90-95 grainers considerably faster than the 105-108 grain bullets at equivalent pressures. In an Eliseo R5 Tubegun, with Broughton 27.5″ 5C barrel, we were able to push the 95gr VLDs a full 160 fps faster than the 108s. This means that the true ballistics of the 90-95s rival that of the heavy bullets — at medium ranges.

We were able to drive the 90-grainers and the 95gr VLDs comfortably and very accurately at 3050 fps, whereas we maxed out at about 2890 fps with the 105gr and 108gr Bergers. At 300 yards, the 95gr bullet’s speed advantage compensates, in large part, for any BC shortfall compared to heavier bullets. In fact, in our rifle, the 95gr VLD actually shows less wind drift at 300 yards than either the Berger 105 Match Target BT or the Berger 108 Match Target BT. See chart.

Here’s data from JBM Ballistics, using G7 Coefficients (500′ alt, 70° temp):

LESSON: Don’t always assume that the heavier bullet has superior ballistics. You have to test, find the accuracy nodes for each bullet in your gun, and run the ballistics for the velocities you can actually achieve with good accuracy. As above, you may be surprised. In our Eliseo Tubegun, the 90-grainers shot tighter than 105s and we gave up little, if anything, in wind drift at 300 yards.

Great Accuracy from 90s and 95s in 6mmBR Tubeguns
In our Broughton-barreled Tubegun, the most accurate bullet so far has been the 90gr Lapua Scenar. In a Savage 6BR with 3-groove PacNor Barrel the Berger 95gr VLD has been ultra-accurate. But we really want to try the 95gr Sierra MK as well. Forum member Randy (aka “InfantryTrophy”) has been shooting the 95gr SMK with great success, and impressive accuracy. Here is his report: “The 95 SMK shoots great. I have not had the opportunity to shoot the 95s at 200 or 300 yards, but I can’t think of anything better to use. This is my first 5-round group fired after about 15 break-in rounds. The load is 29.5 grains of Reloder 15 with SMK 95. The gun is an Eliseo R5 with Pierce action and Broughton 27″ barrel. Shown below is a 5-shot, 100-yard group shot at 100 yards on MR31 target with iron sights, from rest.” Randy measured his group at 0.214″. It looks a bit bigger than that to us, but it is still impressive:

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