Pellet velocity variance
by B.B. Pelletier
This question came in last week, and I thought it was too good to pass up. I’ll try to address it in this post, though I think this one could keep us talking for a while.
“For an unregulated PCP, how consistent does it have to be in order to be good enough for FT at 50 yards? (ie: what’s the maximum standard deviation in FPS allowable considering pellet weights are equal?).”
I used to think that way – that pellet velocity was a key to accuracy. Now, I’m not so sure. This reader has also made the assumption that a precharged rifle that doesn’t have a regulator will have a greater velocity variation than one that does. Usually, that’s true…but not always.
A regulator controls the air pressure that goes to the firing valve. It steps down the reservoir pressure to a level at which the valve operates uniformly. A regulated gun has a chamber between the reg and the valve that contains the optimum amount of air at a given pressure (the pressure at which the reg is set). When the valve opens at firing, it always passes the same volume of air at the same pressure. Regulated guns usually keep their shots at a velocity that doesn’t vary by more than 10-15 f.p.s. I’ve heard stories about guns that never vary by even one f.p.s., but I’ve never seen one.
When the reservoir pressure drops below the pressure at which the reg is set, the gun goes “off the reg,” and the velocity varies more and starts dropping. The big advantage of a regulator is not the tight velocity variation; it’s the greater number of shots a gun can get before needing to be topped off.
Let’s take a look at a balanced valve. Actually, all precharged valves are balanced, more or less, against the fill pressure of the gun. Some, like the Career 707 set on high power, drop in velocity from shot to shot, while others, like a Daystate Harrier, can get a number of shots (mine gets 24) with a tight velocity variation (15 f.p.s.) before they need to be topped off. My Harrier has to be filled to only 2,650 psi instead of 3,000, which makes pumping a lot easier.
I believe the best example of a balanced valve is found in Mac-1’s USFT rifle. It has a fill pressure of 1,800 psi and gets 60 shots while using only 400 psi of air. It shoots a 10.6-grain Beeman Kodiak at 900 f.p.s. with a velocity variation of 15 f.p.s. for all 60 shots. You may notice that it has a huge air reservoir. That’s what it takes to get that many shots at that low pressure. This unregulated PCP has competed successfully at the national level. The starting price for one of these limited-production airguns is $1,800.
The USFT rifle gets a lot of shots with a tight velocity variation on relatively low air pressure.
So, the regulator isn’t needed for shot-to-shot consistency. It’s there to give more shots per charge. However, regulators are failure-prone. It’s not a question of “if” they will fail, but when. If you can get along without one, you’re better off, in my opinion.
The answer to your question is…
Finally I’m getting to the question the reader asked! And the answer is: I don’t really know, but the number is higher than most people think. Remember that Career 707 I mentioned? It might drop 50 f.p.s. in the first 5 shots at high power, but it can still group five Crosman Premiers in half an inch at 50 yards on a perfect day! So is 50 f.p.s. the magic number? Like I said, I don’t know.
I always look for a maximum velocity variation of 20 f.p.s. in a spring gun. If it can do that, I’m happy. Obviously, a spring gun does not have velocity drop-off, but I guess I have transferred that number over to PCPs, as well. I tend to think a PCP that varies by no more than 20 f.p.s. is doing well. When I shoot a PCP to determine the maximum number of shots, I usually make the cutoff at the point where the velocity varies by more than 20 f.p.s. My Harrier is an exception, because it goes crazy after the 24-shot cutoff.
My numbers are arbitrary and mean nothing without shooting the rifle at distance. If you plan to shoot field target, the distance is 55 yards.
I know a field target shooter who was so concerned about velocity variation that he put TWO regulators in his FWB P70 FT rifle! One reg controlled the air for the other! Now that’s as anal as it gets, yet this guy was a nationally ranked shooter. He also mounted a wind gauge on his rifle! I would make fun of him; except that, on my best day, I couldn’t shoot as well as he did on his worst day. On the other hand, there was another man who spent even more money on his gear than Mr. Anal. His custom-made scope rings cost $500! Yet, I could beat him, sometimes. And that, I think, is the real answer here.
In the final analysis, it isn’t the regulator or your equipment or your barrel’s pedigree that matters. It’s how well you shoot. To win at field target takes a rifle that can compete and a shooter who shoots better than most snipers on municipal SWAT teams. You get there by practice, and practice, alone.