Home Forums Hunting & Shooting Ask The Gunwriters Soft primer strike affect velocities???

Anyone ever experience a change in velocities after adjusting how hard the firing pin impacts the primer? I have a Kimber Montana that I rebarreled to a 6mm Dasher. This rifle always had a light strike on primers and even sounded soft while dry firing. With the Dasher barrel I would get misfires once in a while as a result. After adjusting the spring tension on the firing pin, I now get better primer strikes, no misfires, and dry firing sounds “normal” to me now compared with my other rifles. The average velocity of my load has gone up by about 65 fps with similar ES, although the temperature is colder now. I developed this load in 20 deg celcius and have been shooting it now in -3 to -8 deg celcius. This is with the same load components all from the same batches as before the adjustment. H4350, 108 led-m, Lapua brass and cci450. I’m curious to see how the speed will look at 20 deg celcius after the firing pin adjustment. I usually don’t get much speed change with loads using H4350 and at these temps.

I haven't done it in a rifle, but in revolvers I've seen where a light hammer spring resulted in very inconsistent ignition and wide velocity variation. Moving back to a heavier spring brought variations back to normal. I didn't use enough rounds to get a good handle on average velocity though, but wouldn't be surprised if it dropped a little with the lighter spring.

Yes, it can affect velocities--and of course pressure.

One of the big pressure-testing labs I deal with has one guy who's good at "pulling the string" HARD on the piezo setups. His velocity/pressure results are consistently higher than anybody else's.

One thing I have always wondered about when chronographing during different seasons is whether and how much temperature affects the chronograph itself. I think most modern chronographs probably use a crystal controlled oscillator for a time reference, and those tend to be quite reliable and stable, but temperature extremes are bound to cause a bit of variation. I don't recall ever having seen this aspect of chronographs evaluated or discussed.

Have had chronographs "freeze up" in really cold weather, around zero Fahrenheit: They didn't quit displaying, but just kept repeating exactly the same number. A few minutes inside a pickup cab with the heater made things right. (Have never seen anything like that in warmer temperature up to 100+.)

Other than that have never experienced any problems, evidenced by very consistent velocity readings (but not exactly the same!) from loads using temp-resistant powders.

Martin Hagn told me, if he had a rifle which tended to shoot vertical strings, he would first change the striker spring and this usually fixed the problem. Now, I'd been shooting competitively for many years and had built a bunch of accurate rifles and had never thought of this being quite so critical. As it happened, at this time, I had a Model 70 prone rifle in 308. It would shoot Lake City Match into groups, prone with a sling, at 300m, which were about 2 1/2 inches wide but about five inches tall. I put in a new striker spring and was rewarded with nice, round, moa groups. I was, frankly, embarassed that I had to be told. Anyway, later on, I chrono'd rifles under the same circumstances and the differences in consistency was marked. GD

RiverRider
That's a valid concern, and I can't speak for all chronographs. Oehler chronographs all use a crystal oscillator for which the temperature effects are less than the temperature effects on the length of a stainless steel tape or ruler. In other words, don't worry. The effect is there, but is insignificant. The "freeze up" at approximately 0F noted by Mule Deer is usually caused by the battery failing. One writer keeps his chronograph happy at subzero temps by letting it share his handwarmer.

Ken, I did notice that one time with my 35P, put a 9V that was in my pocket for a couple hours and good to go !!

Ken,

Thanks for weighing. I do try to keep batteries pretty warm in real cold by using two reasonably fresh ones, one inside my coat to rotate with the one inside the chronograph--but have still occasionally found it necessary to warm up the chronograph inside the pickup cab. Though the chronograph hasn't necessarily been an Oehler. (I do test others as well, as a responsible journalist should....)

In the past couple years have been doing the cold-weather testing from a roofed, 3-sided "shooting shack," which seems to help, even though I always pick calmer days for cold testing.

Thanks for chiming in, Ken. It's good to know that my 35P will tell me the truth in both hot and cold conditions!

Thanks for the replies. The velocities were measured with a Labradar. I’ve used this Labradar in temps down close to -20 deg celcius and always seems to be consistent.
If this is the “new” pressure and velocity for this load, I may have to reduce it a little. It’s now averaging 2765 fps out of a 20” barrel.