Where do i start

[tech400]

My .177 marauder shot string from new caldwell BPChrony

Where do I start tuning?

Attached is a 40 shot string graph from 2800 to 1800 psi.

I am shooting crossman domes 10.5gr.

Thanks in advance for any suggestions.

Df

[RayK]

The usual goal is to get as many shots within a tight extreme spread per fill.  Your shot string is a strictly decreasing velocity which means too much hammer energy.  4% extreme spread is a good goal.  I tune my 177 to start at around 910 fps, rise to about 945, then drop back down to around 910 fps.

Reducing hammer energy will shift the shot string downward to a lower pressure band.  It will simultaneously reduce the peak velocity of the shot string.  Your Marauder is currently "tuned" for a fill way above 3000 psi which is above what Crosman recommends as the fill limit.  I put "tuned" in quotes because Crosman clearly does not tune their airguns other than something quick and dirty.

You can reduce hammer energy either by shortening hammer throw (turning smaller allen clockwise) or by lowering hammer spring preload (turning larger allen CCW).

[tech400]

Thanks,

I will give it a try.

Can the rifle be under pressure while doing the adjustments?

[Booger]

Thanks,

I will give it a try.

Can the rifle be under pressure while doing the adjustments?

Yes, I would also look up the Bstately mod and consider a Huma Regulator. The best thing I did to every MROD I've owned is to add a regulator. There you have to degass it.

[tech400]

Thank you for your input.

[tech400]

I followed Ray K's recommendation and adjusted hammer spring preload 2-1/2 turns CCW.

Attached is new graph. 3000 - 2200 psi

Comments ??


Average: 878.75
StdDev: 5.79
Min: 864
Max: 887
Spread: 23
True MV: 878.75
Shots/sec: 0.08
Group Size (IN): 0.00

[RayK]

It looks great!

You could leave it there and go have fun, or you could tinker a bit more.

You could fill to 3200 and see how many shots you can get within an extreme spread of 4% of your peak velocity of 887.  4% ~ 35       887-35=852, so you could look for a fill that gives you a velocity starting around 852 then rises to your max of 887, then back down to around 852.  Those should all be good shots for hunting.

If you are doing mostly long range shooting (like 80 to 100+ yards) a 2% ES is better.

If you want to bring your fill pressure down but still keep lots of good shots per fill, you can lower your hammer energy some more and try again.  It will also reduce your peak velocity, but it will shift the good shot string down the pressure band.

Ray

[tech400]

Ray, I am target shooting at 25 -30 yards (property restriction).

I will leave it here for a while. Thanks for your help.

Dennis