Air Arms S400 MPR FT: Part 2
by B.B. Pelletier
Today, we’ll look at the velocity of the Air Arms S400 MPR FT rifle. I’m testing the gun just as it was sent from the factory, which is how I would use it for field target. By looking at the large reservoir tube and knowing that this is a 12 foot-pound rifle, I knew it would get a lot of shots per fill, so today I concentrated on what the gun could do in factory trim.
Blog reader Coax has asked me to try to increase the power, to see what the potential of the rifle is. I’ll do that in a separate report because I don’t want to shortchange today’s lesson. And, a lesson it will be, because the unregulated MPR FT has the classic inverted bathtub curve of power as the air charge bleeds down. I want to talk about that, because it illustrates a couple of important points that new PCP owners need to understand.
Fill the rifle
So, the first thing I did was charge the rifle up to its 190 bar fill point. I used the gauge on my carbon fiber tank, because I’ve learned that it’s usually very accurate. Once the rifle was filled, however, the onboard manometer read less than 180 bar, so be aware that gauges seldom agree.
The Air Arms fill coupling, which I showed back in part 1, requires the rifle to be held in a certain position for the keyed coupler to stay attached. This almost always places the rifle at a most awkward angle during filling and this time was no exception. I wish Air Arms would change to the Foster quick-disconnect that Crosman, Daystate and Quackenbush now use, because that type of coupling allows for maximum positioning flexibility. However, I have to say that once it’s connected there’s no problem filling the rifle.
I want to accomplish several thing when performing the velocity test, and our newer PCP owners would do well to copy what I’m about to do. There are several things we want to learn from this test. First, we want to know the average velocity of several representative pellets. At this point, we don’t know what the most accurate pellet will be in the rifle, but that doesn’t matter. The way I’m doing the test, we’ll still learn everything we need to know; and all that can be transferred to the best pellet, once it’s discovered.
The first step is to fill the rifle to the recommended maximum fill. We’ll soon know whether that pressure, as indicated on our gauges, is correct, or if there are adjustments to be made.
The next thing we want to learn is how many shots there are on a fill. The way I run the test, we’ll be able to select a number of shots from a larger body of data after the test is complete. In other words, we don’t need to know anything going into the test. No preconceptions.
We’ll also note the ending pressure when the rifle falls off the performance curve. If this were a regulated rifle, we’d determine the reservoir pressure at the point when the rifle falls off the reg. That way you can grab the gun and look at the manometer to determine at any time if there are any good shots left in it. That’s handy when you just want to grab the gun for a few quick shots, like a squirrel in the bird feeder.
Crosman Premier 7.9-grain domes
I’ll use the 7.9-grain Crosman Premier domed pellet in the cardboard box as my test pellet for most of this test. I use the lite 7.9-grain pellet instead of the heavy 10.5-grain Crosman Premier because this is a 12 foot-pound rifle. I know that, legally, it has to shoot slower than 825 f.p.s. with this pellet. In fact, it has to shoot a lot slower than that, because the law in the UK says the rifle cannot produce over 12 foot-pounds with any pellet. And, PCPs are always more efficient with heavier pellets. Since the 7.9 is a light pellet, I know the velocity with this pellet has to stay well away from the maximum or risk going over the limit when a heavier pellet is loaded.
Crosman Premier lites averaged 764 f.p.s. in the rifle. Shot one was at 755 f.p.s., but the other nine were at or above 761 f.p.s., which demonstrates the need to “wake up” the valve in a PCP. Whenever I competed in field target, I always shot a couple valve wakeup rounds before starting the match. And, that was a regulated gun! This gun is unregulated, which makes the wake-up call even more important.
The range of the shots was from 755 to 771. That’s pretty broad and perhaps points to the gun being overfilled, but I’m getting ahead of myself. The average velocity produces 10.24 foot-pounds.
JSB Exact heavy domes
The next pellet I tried were JSB Exact domes that weigh 10.2 grains. They averaged 740 f.p.s. and had a much tighter range — 736 to 742 f.p.s. That tells me the rifle is becoming more stable as far as velocity is concerned. That fact will be important in a moment. At the average velocity, the muzzle energy is 12.41 foot-pounds. In the UK, that would be over the legal limit. You see what happens when a heavier pellet is used? That’s why PCPs shoot so slow with light pellets. Of course, they shoot even slower with heavy pellets, so pellet selection is a tricky thing. Of course, you want accuracy over everything else.
Air Arms Falcon
The Air Arms Falcon pellet weighs 7.33 grains, making it the lightest pellet in this test. It averaged 834 f.p.s., with a tight spread from 831 to 836. That’s just 5 f.p.s. — a solid indication that we’re on the power curve. At the average velocity, the muzzle energy is 11.32 foot-pounds, making this a very efficient lightweight pellet that’s certainly in contention as a best pellet for this rifle at this power level.
The next test
After testing these three pellets, I wanted to establish the maximum number of shots I can expect from a single fill. To learn that, I started shooting Premier lites at shot 31. Let me show you the remainder of the shots in this fill
36……..790 (manometer reads 150 bar)
At this point, I’m obviously at the top of the power curve and also at a very flat spot. I wanted to see how the other two pellets would do at this point in the fill/curve. Next, I shot two JSB Exact 10.2-grain domes.
Okay, they’re going even faster than the average we saw before. The string I shot with them was not on the optimum power curve. So, 190 bar indicated on my carbon fiber tank gauge is too high a fill for this rifle. I’ll try 185 bar next fill. Now, I’ll try some Air Arms falcon pellets.
Okay, the Falcons are exactly where they were when I shot the first string of them. For the total useful number of shots per fill, I’ll begin counting with the first string of falcon pellets. I’ll disregard the first two strings, because I now know the rifle was overfilled when I shot them. But the string of Falcons was right on the power curve. Do you see that? Let’s return to the Premier lites to establish the end of the power curve.
76……..771 (manometer reads 125 bar)
I stopped the test here. As a field target competitor, I feel good about the shots from 21 (the first of the Air Arms Falcon string) and shot 75. That gives me a useful string of 54 shots. If I use Crosman Premier lites, I’ll get 54 shots that range between 784 and 794 f.p.s. That’s a 10 foot-second velocity spread for 54 shots. What I would do is mark the place in the match when 50 shots had been fired and fill after that point when it’s convenient. If I were to use Air Arms Falcon pellets, my average velocity would be about 834 f.p.s. for the same 54 shots. My total spread might be well below 10 f.p.s.
Do you see where I’m getting this? All this I’ve learned from just one string of shots. And, that, my friends is why owning a chronograph is so important for a PCP shooter. Not to see how fast you can go, but to learn useful things about your airgun.
Next, I’ll attempt to adjust the power and give you the results of that.