Unintended consequences of riflestock design

Monday, September 08, 2008 Filed in: General gun stuff, Rifles

This weekend I was working around the farm on a particularly labor-intensive project. It got to be about noon, and the rapidly rising temperatures (there was no shade where I was working) convinced me to take the afternoon off and go shooting.

I decided to take my "sport utility rifle", which is a .22LR Marlin 39a. This is the gun that stays loaded all the time, as a .22 goes with farm livin' like beer goes with NASCAR. (I neither drink beer nor watch NASCAR, but Jeff Dunham says so and that's good enough for me.) I'd recently replaced the bead front sight with a plain front post from Skinner Sights, and wanted to see if the new sight picture would significantly improve the usable accuracy.

Along with the rifle and it's usual ammunition, I took some smallbore targets and a few paintballs. (There was a recent thread over at RimfireCentral forums about shooting "fun" targets, and paintballs were a common choice. I don't own a paintball gun, but I now own a box of paintballs!)

After setting up the bullseyes I flopped down to a solid, comfortable prone position and fired my first two groups. I've been shooting iron-sighted target rifles for the past few weeks with great success, so when I walked down to check the target I was stunned at what I saw. Both groups were about three times the size I expected, and centered about an inch-and-a-half high and about the same amount to the left. Well, at least I was consistent!

Keep in mind that this is a gun that gets shot regularly on the plinking range, and never has it shown any tendencies such as I'd just seen. I decided that it was me, and if I did something else for a little while and came back to the rifle I'd be fine.

When I picked up the rifle a half-hour later I decided on a "quick and dirty" test: I'd shoot a few of those little paintballs (which are just a tad over a half-inch in diameter) from the 25 yard line. I set up the bright spheres, took a solid kneeling position and started shooting. The first shot connected and produced a nice orange mist; I pulled the second shot, but the next connected; the last two went just as planned - two more dead paintballs.

This was odd: I could hit these half-inch balls consistently, but if they'd been paper targets I'd have missed completely! It must have been me after all. I flopped down to prone to re-shoot those groups.

Imagine my surprise when I again found two-inch groups, high and to the left! What in the world was going on? Position obviously was a factor; I reshot the groups, this time from my kneeling position. Perfectly centered, and less than half the size of the prone shots.

After thinking about it for a while, it became clear that the problem was a sight issue. The receiver peep sights I have on the gun work better the closer one's eye is to the aperture (which is true with any peep sight.) The further back the eye is from the peep, the less effective that type of sight is.

The design of the Marlin's buttstock was preventing me from getting my eye sufficiently close when prone, but not so much when my body was more upright. The comb of the stock is a bit low, and the point is quite narrow and far back; when in a normal, unstressed prone position it put my eye further back from the aperture than is optimal.

The result was that the "self centering" aspect of the peep sight was reduced, and the depth of field (sharpness about the front sight) was reduced as well. This caused my groups to open up and shift. I found that if I contorted my prone position I could get my eye a bit closer to the sight. That helped with the sight picture but the resulting muscle tension made it impossible to hold steady on target, making the situation even worse.

The ironic part of this is that, had I been using the open sights the gun came with, it wouldn't have been an issue. Eye position is not a factor with the notch-and-bead sights the factory puts on the gun. By putting on the receiver peep sight, I'd changed the interaction of the various parts of the gun's design, and the weakness appeared.

The Marlin stock is great for snap-shooting; looking at it next to a shotgun, one notices similarities in shape and dimensions. Both are designed for efficiency in upright shooting positions, but are less than optimal when the upper body moves to a horizontal plane. The folks who designed the 39a made a great gun, and by introducing a new sighting system I'd bumped into the limitations of their design.

This episode has helped me understand how the elements of a rifle stock design interact with the shooter. I already know (from hard experience) that a Monte Carlo stock design has serious problems with certain shooting positions (particularly in prone), but I hadn't stopped to consider all the other little intricacies.

Even after 40-plus years on this planet, I learn something new every single time I go to the range!

-=[ Grant ]=-

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