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June 21st, 2007

6mm Bore Diameter and Bullet Selection

Long-time visitors to this site will recall that, in 2006, Jackie Schmidt did an extensive 6BR accuracy test with his rail gun. It was fitted with a straight, heavy contour .237″ land-diameter Krieger barrel. Many interesting discoveries came from that test, including the relationship of barrel length to velocity. Jackie cut that barrel down in 1″ increments from 33″ to 28″ and we only lost 8 fps per inch.

That test barrel also showed a very strong preference for Berger 105gr VLD bullets, which (at that time) measured about .2433″ at the pressure ring. To our surprise, however, the accuracy of Lapua 105 Scenars was good, but not what we anticipated based on results in other rifles. We noted that the Scenars were extremely uniform in weight and dimensions. However, the max bullet diameter was around .2428″. So these Scenar bullets were about half a thousandth skinnier than the 105gr Bergers.

Lapua 6mm 105gr Scenar

Can the Scenars shoot superbly in a Krieger? The answer is yes, if the Krieger is a .236″. Bore dimension (specifically land diameter) DOES make a difference, based on recent results from Jackie.

Jackie has recently learned that the Lapua 105 Scenars WILL shoot with blistering accuracy in a Krieger barrel–a Krieger .236″ bore barrel. Jackie told us: “Well, I have to admit it… you guys were right about those Lapua Scenars. I just built up a long-range 6BR rifle for a friend. We put a Krieger 4-groove .236″ barrel on it, and, man those Scenars shot like a house on fire. The accuracy is just amazing. After my experience with the rail gun project I never would have believed it. But yes, the Scenars are outstanding in a .236″. I personally shot a measured .270″ 5-shot group at 200 yards with the Lapua 105s in that gun. And the velocity–you wouldn’t believe it–we were seeing 3000 fps with a full case of Vihtavuori N150. In a long barrel, with a .236″ bore, the N150/Scenar combo is a winner–for accuracy and velocity. You just have to match the bullet to the barrel. Skinny bullet? Go .236″. It works.”

Permalink Bullets, Brass, Ammo 5 Comments »
June 21st, 2007

Scope Boosting–Sightron vs. Weaver

One thing that Jackie Schmidt has been doing over the past few months, is dissassembling Weaver and Sightron 36X Benchrest scopes and locking them up internally. With the internal windage and elevation controls fixed in place, these scope are then mounted using the externally adjustable scope mount system developed by Gene Bukys (shown below).

Gene Bukys Scope Mount

In working with the different scopes, Jackie has observed that the basic designs of the Weaver T-36 and the Sightron BR scopes are similar. However, Jackie did find some important differences when it comes to boosting scope magnification. The Weaver is a better candidate for boosting because of the way its reticle is installed.

Jackie writes: “I have had both brands apart, and they both share the same design feature in the way the erector tube is mounted, that being a permanently fixed ball and gimble joint sleeve assembly that allows the erector tube to pivot. There is a difference in the way the erector tube sleeve is secured inside the scope body, but that is of no consequence. They both share a similar front objective assembly, (sliding lens assembly cartridge held against a stop by a large coil spring), so there isn’t much advantage of one over the other. I would call it a toss up as to which one might be better, or worse. The Weaver is a couple of ounces lighter. The one difference is if you wanted to boost one or the other. When boosting the power on a scope, one must keep the reticle cross hairs within the focal plain of the eye piece lens assembly. Most scopes are boosted by adding a small lens assembly between the erector tube and the eye piece, moving the reticle assembly back so as to maintain its correct position in relation to the eye piece. Since the Sightron has the reticle mounted in the end of the erector tube, it would be very difficult to move it back. The Weaver is much like the older Leupolds, you can move the reticle assembly back, since it is mounted into the scope body itself.”

Permalink Optics No Comments »