Efficiency For me, the "magic" of the USFT comes from its low fill pressures. This post will address this novelty in the PCP world.
When most of us take the leap into PCPs we find ourselves questioning how we're going to fill them. If it's been too long for you to remember that stage of the hobby, just take a look at any given moment on any of the forums and you'll find guys asking about pumps, compressors, tanks, fittings, fill stations, etc, as they try to figure out what is going to work best for them and their needs and wants. Occasionally we get somebody asking about 3000 psi SCUBAs, and very frequently we get guys asking about hand pumps. The typical responses to both are "NO WAY, can't even fill a PCP one time with a 3000 SCUBA," and, "ha ha, enjoy the hand pump to 250-300 bar, snort." For most guns, those responses are fairly appropriate as you truly can't fill a PCP with a +3000psi (207 bar) max fill pressure even once with a 3000 SCUBA tank, and hand pumps do really suck (suck the fun out of it) for filling high pressure guns. And the trend seems to be moving towards even higher and higher fill pressures.
Enter the tubed version of the USFT. There's a slight variation in the fill pressure that most tubed USFT's prefer to run at, but most that I've been around (mine and friend's guns) seem to like to fill up to around 1500psi. My personal example does really well with filling to 1450psi, and shooting down to 1250psi. A PCP operating in those pressure ranges is truly an oddball in the PCP world. For example, consider that many guns have 250 bar fill pressures (3626psi) and the USFT fills to only 1450, that's a difference of 2176psi! So, it's got to be a weak little 6fpe-producing 10m gun right? Nope. Mine does right under 20fpe to make it field target legal. LD recently shared here on AGN somewhere that he shot a highly modified USFT up at the long range airgun competition in Oregon a few months ago. That was a .22 shooting the MRDs (25.4grs) at around 50fpe if I remember the details correctly, and still a low fill pressure gun. I can't remember the top end he was filling to, and he may not have even said, but really impressive to get that kind of power from such a low pressure. (LD, feel free to chime in with the details on that Oregon gun, and I'd love to hear your secrets on to get such high power from such low psi.)
I've been meaning to shoot a current shot string over the chrono but have just been too busy so I'm recycling the #s from this post:
https://www.airgunnation.com/topic/usft-unregulated-shot-string/ I don't expect much variation in current fps numbers from those that I shot a few years ago, as I haven't needed to touch much. I very vaguely remember giving the transfer port resctrictor something like 1/16th of a turn "in" but I'm still using the same dope sheet from this old string and all the impact points match, so it didn't change much. Maybe I'll still shoot a string over the chrono if I get time, just to see what's changed since 2019.
Here is the breakdown from that recycled 2019 string.
I intentionally overfilled, and shot down past the 1250psi, just for the sake of collecting data. (The break in the plotted line was an error in the chrono).
The gun is good for a very consistent 40 shots and actually more than that throughout a match as the temp increases. I've shot 48 shot matches without refilling. The rising temp throughout a morning increases the tube pressure enough to get some extra shots. I've also had the opposite happen to me, with decreasing temps the gun will lose a few shots. The important thing is that I know if the gauge is between 1450 and 1250, I can 100% trust the fps to be consistent enough to have complete faith in where the pellet is going to impact at various distances.
Here's the efficiency numbers, not too bad, 1.28fpe/cu inch per shot-puts it in Lloyds "good" category. (thanks to Lloyd Sikes for making these calculators available for general public use).
So, how big is that tube?
The air holding portion of the tube looks to be 19-20.5 inches long. I'm not sure where the end piece stops at the rear of the tube.
Outer diameter of mine is about 1.995in or 5.07cm.
Arzrover measured the inner diameter of the tubes to be 4cm. That gives wall thicknesses of just a bit over 1cm if you're curious. And using good ole V=(pie)(r squared)h for volume of a cylinder, we're looking at a volume of around 640ccs (using radius=2cm and height = 50.8cm or 20 inches).
640cc giving 40 good consistent shots from 1450 to 1250psi.
I started out thinking this was going to be my last PCP (lol to that one, 5 or 6 other PCP's later), so I invested in a heavy 80cf, 3000 PSI SCUBA tank. It proved to be too big and heavy for mobility so I bought a slender little 19cf tank, also 3000 psi, to use as a sort of buddy bottle for my USFT. Eventually I wanted some more PCPs so ended up with an 87cf SCBA carbon fiber tank too.
Here are the numbers for those 3 tanks.
The little 19cf SCUBA is about 4 inches in diameter and about 19 inches long. It's about the size of a silver and green oxygen bottle that you see people carting around. With that little 19cf tank I can fill the gun 31 times, @ 40 shots per fill, that's 1240 shots, or, nearly 2.5, 500 count tins. LOTS of shots.
The 80cf SCUBA gives 130 fills, @ 40 shots per fill, that's 5200 shots, or nearly 10.5, 500 count tins. LOTS and LOTS of shots.
The 87cf SCBA gives 186 fills, @ 40 shots per fill, that's 7440 shots, or nearly 15, 500 count tins. LOT and LOTS and LOTS of shots.
Now, that's all theoretical numbers, and I've never ONLY filled the USFT from the big 80cf SCUBA or the SCBA so cant verify anything there, but I have filled ONLY the USFT from the 19cf smaller SCUBA and I can verify that it will shoot more than 2 tins of pellets through it before the pressure gets down to lower than 1250psi. So, the #s LLoyd's calculator predicts for the bigger tanks seem plausible.
What all that means, is that for somebody who doesn't need more than 20fpe, the USFT can shoot and shoot and shoot and shoot with one fill from a simple ole 3000psi SCUBA tank. This makes the chasing of high pressure compressed air much more simple, as true 4500 psi fills in a SCBA are harder to come by than 3000 fills. And if you own the compressor, 3000psi fills are much less wear and tear on equipment than 4500psi fills.
Furthermore, many of the male fosters on my higher pressure guns eventually end up with ball bearing divots from the inside of the female foster and the pressure exerted on the female foster shoulder. The male foster on my USFT has none of that nonsense, and I don't expect it to ever get that deformation, as 1450psi is simply easier on all the fittings.
And there's no regulator to ever act wonky. Wink wink.
A true oddity in the airgun world.
(For those hand pump guys...I did fill the gun one time with a Hill MK3 hand pump, shortly after getting it. From memory it was something like 120pumps from 1250 up to 1450psi, and OH SOOO easy strokes. I used to fill a Benji Discovery up to 1900 with a hand pump and the pump strokes for the USFT made the pump strokes for the Disco seem hard, and even they're not too bad compared to trying hand pump a 250 bar PCP. So, VERY hand pump friendly gun, the tubed USFTs.).
