Crosman MK I HPA Project

[paddyfritz]

Nice job..

[sholo]

Well..it has taken forever, but I finally got it scoped and running on HPA!

First thing I noticed was how shaky I am with a pistol! Thought I was having a siezure at first!

With the regulator set at 850 psi, stock hammer spring, I got 120 shots @ 553.5 fps (avg). Highest=555.9, lowest=551.1. Just out of curiousity, I tried 5 shots with 16 gr Polymags (heaviest pellet I have at the moment) and averaged 529.1 fps. I'm really happy with how consistent the fps #'s are - max 4.8 fps differential over 120 shots!!!

In order to get to the required sub-500 fps mark, I have a couple of options as far as I know:
1) Keep shooting past the 850 psi set point until I reach my goal (495-499 fps), measure pressure and adjust regulator accordingly, or
2)Keep reg at 850 psi, and clip 1/2 - 1 coil at a time until goal is reached.

I think that seeing as I have a couple of stock hammer springs, I may do option 1 first, then do option 2 and compare shot counts to see which one is more efficient. Better stock up on pellets!

As for accuracy, I was pleasantly surprised at how accurate it is, considering it is a pistol (and more importantly - me pulling the trigger...). Scope I'm using is an NcStar 2-7x32 (10.5" eye relief for my ape arms) on CenterPoint medium height Weaver rings, which, to be honest, is not too bad at all for a $70 scope. I just wish it had a Mil Dot reticle and AO... Scope rail is from a Thompson Centre Contender (fits shroud perfectly).

Was rainy/drizzling out today, so I was confined to the basement for some target practise (10 yards). After practising different holds/controlling my breathing, this was my best grouping so far:

Not world class by any means, and granted, it's not at 30-40 yards, but pretty gratifying for me, who has never been very good with a pistol!
Out of the 4 pellets I had to choose from (CPHP, CPUM, Benjamin Discovery HP, and Predator Polymags), the Benjamin's were the best, with the Polymags a close second. Can't wait to try some JSB Exacts...

Todd

[Doc Sharptail]

Nice work. You may notice a bit of a "steady down" by going to 1 X Red Dot. I've found that it helps to cure me of the magnification fight- on a pistol. A pistol is a heck of a thing to scoperize- unless you're going to be doing all your shooting from sitting, 2-handed over the knees, or from a rest. I don't mean to dissuade you from the pistol scope- if it gives you confidence, by all means, go for it. The steadier sight picture afforded by the Red Dot is a confidence booster for me. It helps me to shoot in a bit more relaxed fashion.

Nice to see the moderate velocity, and high shot count together. You may have one more option for velocity reduction- the jsb 18.1 gr. Exact heavy pellet... I haven't tried the Discovery pellet in .22 yet. In .177, they're not bad, but I've found the 10.1 gr ultra mag to have a slight edge in accuracy...

Regards,

Doc Sharptail

[sholo]

Thanks for the tips, Doc! I may try the Red Dot if I can't keep the scope under control, but I'm still in the early stages yet, so I'll give it a bit of time before I make up my mind.

I thought about the 18.1 gr JSB's, but seeing as it will be primarily a target gun, I might try and stay with 14.3/14.5 gr, and hopefully gain some more shots when all is said and done. Having said that though, I will try the JSB's before changing anything, just to see what #'s they produce.

[sholo]

Got the day off work yesterday and thought I could get some more testing done, but the dreaded "Honey Do" list kept getting in the way. (Damn list is longer than I thought!)

Anyways, it was after supper when I finally got to work on it . With the regulator set at 850 psi (all disc springs in place, no shims), it was over the 500 fps limit...avg 553.5 with 14.3 gr CPHP. So I drained bottle, took regulator apart, and removed 1 disc spring and added 1 thick shim (.020"), which gave me 448-450 fps. Pressure with this set up was 425 psi.

Sooo...took reg apart again, added 1 thin shim (.005") to the thick shim already in place, and it shot @ 481 fps, pressure was 525 psi. Roughly a 30 fps gain.

Took reg apart again (getting pretty good at this now!), added 1 more thin shim, and shot 506 fps (10 shot avg, 3 fps deviation). pressure was now 625 psi. Oh, so close!

I want to be as close to 500 fps - but not over - using 14.3 gr ammo so that if I decide to use heavier pellets, i.e: 15.9 gr JSB Exacts, I won't be too far from the 500 fps mark (475?). I think I may leave the regulator set at this point (625 psi), and turn my attention to the hammer spring for the final tweaking. I think that I would only need to clip 1/2 - 1 coil in order to reach my goal. That will be tonight's chore after work, then hopefully do a shot string...

Todd

[rsterne]

You will find that adding hammer spring past a certain point buys nothing in velocity.... On the other hand, having too light a hammer strike will result in the velocity increasing below the regulator setpoint.... Check out this thread.... http://www.canadianairguns.com/showt...-Hammer-Strike

You best balance between performance and shot count will occur when the hammer strike is balanced against the pressure just as if you have a non-regulated PCP running right at the top of the bell-curve....

Bob

[sholo]

First off, I should point out that regulator set point pressures in my previous post should have read 450, 550, and 650 psi - not 425, 525, and 625 psi. Sorry 'bout that...

I decided to stay at 650 psi output pressure as this was the closest I could get to 500 fps by adjusting regulator pressure alone (506 fps). I then clipped 1/4 - 1/2 a coil from hammer spring, and landed right where I wanted to be: 492 - 497 fps. Sweet!!!

So..I went for a full 3000 psi fill and subsequent shot string, but had a wee problem when I got to 2500 - 2700 psi:

DAMN!!!

By the time I got pump disconnected, and bottle attached to gun again, I was down to 2100 psi. Oh well, enough pressure to compare with the 850 psi set point I originally started with.

Shot #-----High-----Low-----Avg

1 - 40 ------498.0-----492.2----495.1
41-80-------494.7-----488.7----491.7
81-120------491.1-----486.2----488.7 *reached regulator set point of 650 psi*
121-132 dropped about 2-3 fps per shot (484.6 - 459.8)

I got 120 shots within 12 fps of peak fps (498.0 - 486.2 = 492.1 avg) using 14.3 gr CPHP as my test ammo.

The same # of shots as a 3000 psi fill @ 850 psi output, but with 900-1000 psi less fill! If my math is right (?), this should equate to roughly 170 - 180 shots in the 492 - 495 (avg) fps range.

I contacted Eric Thursday night regarding fill hose, and had a new S/S braided fill hose in my hands Friday at noon! How do you beat service like that? Thanks Eric, top notch as usual!

Now to do a full 3000 psi fill shot string and accuracy test @ 25 yards...

@ Rsterne: thanks for the advice regarding too light a hammer hit at a given pressure, gave me something to think about. Seeing as fps #'s never did rise as I approached set point, and gradually declined immediately following, I think I got it right (?)...


Todd

[rsterne]

Since the velocity never increased below the setpoint, the hammer hit is not too light.... If it drops off immediately at the setpoint it may, however, be heavier than needed to achieve the velocity you can at that pressure.... The fact that clipping 1/4-1/2 turn off actually reduced the velocity makes this unlikely, however.... If the hammer hit is too heavy, all you are doing is losing shots.... so if you are happy with the shot count I'd leave it alone.... Also, you have the velocity right where you want it.... Well Done !!!

When you get a full 3000 psi fill, you might want to check the velocities again as you might be a whisker over due to regulator "creep"...

Bob