I really enjoy going out on the weekend and doing a little target practice with my Ruger 10/22 rifle. The el-cheapo .22lr ammo I buy is really inexpensive (averages $.04 a round) so shooting for a few hours is not very painful in the wallet. The problem is that my cheap ammo isn't the best quality and I often get a few flyers every one or two magazines. I'll be shooting dead on and then all of the sudden my bullet will hit an inch away from the target for no reason. Now, it could be that I jumped or blinked or the wind picked up but I'm convinced that it is being caused by a crappy bullet or two. I've seen several people talk about rim thickness gauges that are designed for .22lr but they are $30-$70 and I didn't want to pay that much for one so I decided to build my own. The theory is that rim thickness affects how deeply the cartridge sits in the chamber. That difference makes the bullet engage the lands and grooves at different times giving you to great a variance in lock time/return to battery. This also affects the head space.
My thoughts were to drill a hole in something of a fixed length with a hole large enough for the casing but not the rim. Then I could measure the difference in length with the round in and out of the hole. The difference would be the thickness of the rim. At first I thought an old bolt would be good material to use but as I looked around the garage I really couldn't find one that was suitable. I did, however, find an old (have no idea what this thing is called - it is used in machine shops and I will put the real name here sometime in the future) that my wife's grandfather gave me in a box of junk. The machining was pretty good on it so it was a nice, square stock to use:
I set it up in a vice on my drill press so I could drill the hole true. I used a 15/64" drill bit:
The white stuff in the picture above is WD40. I figured it would keep the bit from buring up since I wasn't really sure how hard the material in the block was. After drilling there was a sharp rim around the hole that would have kept the round from sitting flush on the face of the block so I used a 1/2" drill bit turned slowly in my hand to cut the rim off:
With the hole drilled to 15/64" the round can be placed in the hole but the rim keeps it from falling through:
Here is a bullet inserted into my new thickness gauge:
I used calipers to measure the thickness:
I grabbed the remainder of a box of Federal Bulk Pack (the stuff in the red box you get at Walmart) and started measuring. After measuring, I sorted the rounds into piles based on thickness. There were 292 rounds in the box and the vast majority of the rims were 0.038" thick. The breakdown of thickness count was:
0.034" - 002 - 01%
0.035" - 011 - 04%
0.036" - 002 - 01%
0.037" - 032 - 11%
0.038" - 178 - 61%
0.039" - 067 - 22%
Note that most of the ones I had marked as 0.039" were actually 0.0385" and likewise with the 0.037" ones. I really need digital calipers if I am going to do a better job of sorting them. I figure that ~75% of the rounds were 0.038 +/- 0.0005. The 15 rounds out of "spec" on the low end are what I am thinking is causing the flyers that I am seeing.
After measuring the rim thickness I took the 178 rounds (61% of my original sample) that were 0.038" and weighed them on a digital scale. The results were:
3.11g - 05 - 02%
3.12g - 17 - 17%
3.13g - 77 - 77%
3.14g - 75 - 75%
3.16g - 01 - 01%
3.26g - 02 - 01%
3.29g - 02 - 01%
3.30g - 01 - 01%
I see that 94% of the rounds were 3.13 +/- 0.01 grams. It will be interesting to see if throwing everything out except the 169 rounds that were in spec as far as the rim thickness and weight improves how the rifle is shooting. I'll report the results in a week when I go shooting again.
UPDATE - I went to the range today to run a quick test on the matched ammo. I placed a paper with dime sized targets 25 yards away (indoor range) and shot 30 rounds to warm up the barrel. After that 30 rounds I fired 15 rounds that were all 0.038" and 3.13 grams then 5 rounds that were 0.038" but weighed 3.26g to 3.30g (they were the heaviest 5 rounds from the sample taken above). There was no doubt that the first 15 shots were better than the last 5 shots. No fliers were noticed in the first 15 but the last 5 contained three that hit low left (as much as 1" off center). So, what does all this mean?
1) Sorting cheap ammo by rim thickness/weight (this was actually just a weight sort) does appear to make a difference in consistancy.
2) Sorting ammo by rim thickness/weight might take longer than what I really want to spend prepping for a day of shooting.
3) I need to do further testing so I can see the results of sorting just on rim thickness. Maybe all I need to do is weigh the rounds which would save a lot of time.
4) More expensive match grade ammo should be used when betting lunch on who has the tighter groups.
5) I like being able to hit the bullseye every time. It could be time to invest in some further Ruger 10/22 modifications (bull barrel, better trigger group, etc).
UPDATE #2 - It has been a week since the last test and I wanted to do some more in depth testing with the ammo. I went to the same indoor range and shot some quarter sized targets from 25 yards. This testing was a little more in depth because I shot 100 rounds of the sorted and 100 rounds of the unsorted. The results? Well, I am now convinced that sorting the ammo has very little to do with how it shoots. My original test simply did not have enough data points to get good results. With 100 shots it was plain to see that both the sorted and unsorted ammo shot about the same. I got around the same number of fliers (shots that hit 1/2" or more from where I was trying to shoot) with both types of ammo. In the picture below you can see the unsorted (marked R for random rounds pulled out of the box) and sorted (marked M for measured). Even though the only "horrible" shot in this picture was from unsorted ammo I did have just as many horrible shots (on other targets) that came from sorted ammo. I might buy a box or two of real match grade ammo and see what kind of results I get from those.
Two months after the last test and I finally have better ammo. I purchased 5000 rounds of CCI Standard Velocity to see if it would improve my groupings. I was kind of nervous when I opened the first box because that it a lot of ammo to buy without testing it first but I am now happy with the purchase. The groupings were tighter with the new ammo and you can actually feel the difference when shooting it as well. The ammo has less of a sharp crack and less recoil than what I have been shooting. Interestingly, I had to adjust my sights because the new ammo had a 1" drop at 25 yards compared to the other stuff I was shooting. The shots on top were the CCI SV ($0.044 per round or $240 shipped per 5000 rounds) and the shots on the bottom were the cheap Federal Bulk Pack in the red box from Walmart: