Author Topic: Shot string variances  (Read 186 times)

Alan

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Shot string variances
« on: March 30, 2020, 05:25:05 AM »
I didn't know which forum to post this in, but you are the resident professor of airgun technology! So here goes.

Daystate has started shipping their new Delta Wolf airgun. The suppressor (I think it is the primary function) incorporates an electronic chronograph. They claim the gun has preprogrammed settings for different weight pellets and calibers. However (based on data from users), the electronics require a few shots to be taken, before the velocity settles down to a decent string. When I say decent string, once the "brain" trains itself (if we can call it a brain). What puzzles me a bit, is the strings I've been able to find on-line, show the variance to be about ±8 feet per second. This isn't much better than a well-tuned, and regulated (which it is via Huma) airgun without all of the electronics. I should mention that the consecutive shots seldom, if ever, are the same velocity.

I don't really want to make a comparison here, but it is necessary. The strings from my FX Impact tend to be hovering around a central point (944 FPS in my case), with a few deviated velocities, for a string seldom over ±3 FPS. The only real difference, other than the electronics, is the FX incorporates a hammer stop. As Earnest Rowe points out in his numbers videos, is that the hammer stop is the most critical adjustment on the airgun. Having gone through the tuning process several times, I agree. Again, in my case, an adjustment of just 1/8 turn, evened out the shot string as described above.

It would seem to me, if we wanted to base a new airgun around electronics, you want to do so like Daystate's earlier airguns, and measure the velocity much closer to the chamber, not the muzzle, and perhaps in multiple places. And you'd probably want a much larger plenum than they are currently using.

Any thoughts?


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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.

rsterne

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Re: Shot string variances
« Reply #1 on: March 30, 2020, 10:12:34 AM »
I don't know anything about how the new Daystate (or the older ones, for that matter) works.... I would assume they use a solenoid to control the hammer strike, to open a conventional valve, but that may not be the case, they may use an electronic valve (and I don't even know how those work)…. I responded to a thread on the GTA recently about the resolution required to tune a PCP (electronic or conventional) and for a typical tune, producing 900 fps, and where the valve closes at about 25% of a 20" barrel (ie when the pellet is 5" from the breech)…. To get a ES of 1% (+- 4.5 fps) requires that the valve dwell be controlled within about 0.000025 sec. (0.025 mSec. or 25 uSec.)…. It is hard to believe that our simple, mechanical knock-open valves are capable of such consistency, but that must be the case, given what we know about how PCP internal ballistics work....  :o

To be able to adjust the average velocity by a 1% increment (eg. from 900 fps to 909 fps) requires the electronics, programming and all the mechanical systems to be able to make an accurate and repeatable change of just 25 uSec…. If the "brains" of the gun are deciding what dwell to use, and how much to discharge a capacitor to achieve that, that is the resolution required.... both to achieve the new tune, and to maintain it within a 1% ES.... It has to do this over a wide range of temperatures (of course it can "adjust" itself on the fly, after a few shots, if its smart enough)…. and it has to understand the regulator performance as well.... ie does the output pressure change as the reservoir pressure drops?....

The idea of using a hammer stop as the last stage in tuning a mechanical PCP makes a lot of sense.... The biggest variation we have, IMO, is consistency of hammer strike over temperature and gun attitude change (shooting up or down changes the effect of gravity on the hammer ever so slightly)…. As I understand the concept, you tune the PCP to give just slightly more performance than you want without the stop, so that the hammer strike is just a bit more than required.... Then you use the hammer stop to physically limit the valve lift to a couple of thou less.... In theory a lighter or heavier hammer strike can't change that lift, so every shot is the same, other than for variations from pellet to pellet.... An electronic gun, unless it controls the valve that precisely, and directly, probably can't reach that consistency....

Fortunately, getting the ES smaller than 1% is not a requirement until the ranges exceed 200 yards or so.... For 100 yard work, a 2% ES will be lost in the group size.... a 1% error in estimating the range or a 1 mph error in guessing the wind speed are more important....

Bob
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Alan

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Re: Shot string variances
« Reply #2 on: March 30, 2020, 01:25:40 PM »
Our thoughts are fairly well in line. Although....

My forte is electronics, not necessarily strictly math and physics, and it would take several pages to explain why that is the case. I've put some thought into this issue, not because I'm that curious (well, I am!), but what might be possible if computerization didn't have a cost limit? I'll give you an example.

The Navy's new magnetic rail gun can handle a variety of projectile weights and designs, yet keep the muzzle terminal velocity (≥ 4,500 FPS) within a few FPS. I am sure, no expense has been spared to achieve this. How they do it, I can only imagine. But in the case of Daystate, it would seem that moving the velocity measurement out to the end of the muzzle, rather than about 6 inches in front of the breach, was a mistake. Albeit, perhaps a marking coup?

As you alluded to, it is amazing that mechanically-based, airgun tuning, can get as close to ideal as it is. In some cases, our (airgunners) specs exceed what is possible given firearm ammunition tolerances being what they are. Only Benchrest guys, with the wherewithal, get smaller velocity spreads.
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Alan

I have an Omega compressor. If you're a fellow Guild member, and you pass through Roswell, NM, I'll fill your portable tank as a courtesy.