SpudFiles

When I was recently test firing a long half-inch PVC gun I made the results made me think about how to shoot longe distances accurately with spudguns. I shot at a cardboard box about 40m away, and since in my experience guns generally don't shoot as well as you think they will, I aimed slightly above the box to account for expected bullet drop. However, the bullet went over the box by several meters and flew off out of sight off the baseball field I was shooting on. This aiming error of a few degrees of elevation made a big difference in range and accuracy.

I just finished a prototype of an adjustable angle scope mount for an updated version of this gun. The scope (a $20 4X rifle scope) is mounted on a section of PVC that can be raised or lowered by turning a screw. The mount is calibrated so that a full turn of the screw raises or lowers the angle of the scope by 0.4 degrees. Using the excellent ballistics software by Joanna Ardway from the SpudWiki, I can predict the range of the gun at a given angle. The gun is leak-free and my pump has a good psi gauge, and my ammo is made out of modded nerf darts and screws, both of which are comercially manufactured (aka you can buy a bunch of them and they are exactly the same size and weight). This means that I can get the gun to a precise pressure and shoot uniform ammo. I'm just waiting on the sprinkler valve for the new gun to arrive in the mail, then I'll take it out for a spin. In the mean time, any thoughts/predictions on whether this gun and scope system will be as accurate as I hope? I'm hoping to be able to hit, say, 1 square meter from 100m away.

So your hoping your homemade elevation adjustment is more accurate than the elevation adjust already provided by your scope? Yea, the big knob on the top of the scope is for elevation. Scopes are meant to be mounted solid then sighted in for an average distance that you shoot. Any change in distance(+ or -) can then be adjusted with the elevation knob(up or down). Your scope system is a waste of time and probably won't be accurate.

If this was a homemade open sight setup then maybe it would be worthwile. So many open sights exist with elevation adjustments that is not even a new idea.

m4xr8d wrote:Your scope system is a waste of time

Only partial marks at best I'm afraid.

Scopes have a limit to their internal adjustments, they can only adjust down so far. With a low velocity spudgun or sometimes even an air rifle, the trajectory is so curved relative to a regular rifle that at longer ranges, the scopes can't adjust downwards enough.

That's why quite a number of airgun scope mount sets have a rear mount that can be raised to make it possible to tilt the scope down to allow it to be used at longer ranges. You do the majority of the vertical adjustment on the mounts, then use the vertical adjustment knob to do the fine tuning.

In this case, with a low velocity spudgun, the design might well be necessary.
I'm designing a long range dart, and even though I'll be firing it at about 900 fps, with a G1 ballistic coefficient of about ~2.8 (for comparison, a G1 ballistic coefficient of 0.5 is considered pretty good - 2.8 is off the scale), I'll need to use a higher mount at the rear to compensate for the curved trajectory at longer ranges.

Mind you, long range is going to be into hundreds of yards.

given the fact that you bought a 4X sight (not a 20X scope or something like that) your field of view should be broad enough to allow you to simply aim above the target (it can be done quite accurate if it is a mildot sight) ro combine the aforementioned way with an inbuilt distance adjustment that most scopes have....

instead of having an adjustable rear mount I think it would be better if you simply have a slightly rised rear mount and have your gun zerod for 50 meters or more..
keep in mind it is difficult to build a gun that's accurate for ranges above 150 m (mostly do to the fact that ammo is not stable)

most ammo has such a high drag that the max effective and accurate range is limited to 150 m... even for relatively powerful guns (mind you that you have a 1/2" PVC gun, small diameter of barrel + the fact that PVC can't hold high pressures means that your gun has relatively low muzzle energy... my 1/2" gun runs at pressures up to 500 psi and I have to admit that accuracy and power sucks at ranges greater than 100 m - mostly due to the fact that most 1/2" ammo is either not stable or does not have enough mass to keep its energy for ranges above 50 meters....

so to sum it up... if you want to have an accurate long range gun you should concentrate on finding a high density ammo and build a more powerful gun... then you can do what I or ragnarok said...

Thanks for the feedback guys
@ Poland Spud: If by more powerful gun you mean larger caliber, then how do you get around the increased drag from the greater projectile cross-section? Sabot round? Also, the PVC gun is sort of a trial balloon for a higher pressure copper gun I have in the works, which should be a good deal more powerful.

skyjive wrote:@ Poland Spud: If by more powerful gun you mean larger caliber, then how do you get around the increased drag from the greater projectile cross-section? Sabot round?

More powerful doesn't have to mean larger calibre. You can improve power with longer barrels, more pressure, better valves, and other optimisation.

Of course, sabot rounds are the best of both worlds - the power of a large calibre, but the low drag of a small one.

Skyjive: Don't take folks comments too seriously. The only valid way to judge your setup is by its performance. What kind of accuracy (reproducability) are you getting? What's the dispersion (grouping) at 40m?

If by more powerful gun you mean larger caliber, then how do you get around the increased drag from the greater projectile cross-section? Sabot round?

I didn't say that... I said that your gun has relatively small caliber and it has to be running at low pressures ( <200 psi ) since it's made from PVC...

so your gun could be more powerful if it had larger caliber and/or if it used higher pressures....

what rag wrote is ok

More powerful doesn't have to mean larger calibre. You can improve power with longer barrels, more pressure, better valves, and other optimisation.

but it's mainly about pressure

=Increasing the barrel does improve performance but do you think it is practcal to have a 300 cm barrel on a handheld gun ?

=the best valve you can get is either a burst disk or a QEV valve (or a piston valve) with a slightly larger porting than the ID of barrel

=there are several other things you can do to improve performance - I am sure that rag will explain them to you if you ask him.. but generally they are either expensive or they don't give you much increase in power

=using higher pressures does give you a substantial increase in power. While you do need to use more powerfull materials that can withstand it. Higher pressures simply allows you to get more power without using helium, having a 2 m long barrel or a 5 L chamber


@jimmy

Don't take folks comments too seriously.

if he wants to be able to hit a 1X1 m target at 100m that's achievable but I don't think that a 1/2" PVC gun can be accurate at let say 150 m
most 1/2" ammo are simply too light and have too high drag.. what is more, if it's not stable it simply can't be accurate at longer ranges

POLAND_SPUD wrote:but it's mainly about pressure

Only if you've got a good valve in the first place.

My personal opinion is that a good valve needs to come before high pressure.

For example, if I were to be using something like a modded 1" sprinkler on HEAL (assuming the valve would take 300 psi), I'd be sacrificing a good 20-25% of the power.
Unmodded, maybe 30-35%.

If I can get an extra ~30% performance for free out of the same chamber pressure, that's a good thing to me.

You need both a good valve and high pressure really.

My 3/4" McMaster QEV just came, that's going right onto the copper version of the PVC gun I tested, hopefully it will work well. Plan to pressurize to 200-300 psi with CO2 cartridges.

@ rag
yeah but QEVs are not that expensive and they are slowly becoming a standard...

besides I think he has already taken care of that


now one more thing... what can you do to increase the power when you already have a QEV and you are using it do its fullest potential (meaning that the valve porting is a bit bigger than the ID of the barrel)?
pressure is a different story... you can up it a lot