Pressure, Velocity, and Accuracy

[.22 Varmint]

   I have a Benjamin Marauder in .22, I have owned the gun for a few years and with the help of Tim Mcmurray have found a good barrel for it, Thanks Tim! I finally made it over to the PB range to do some long range shooting. I charged the gun to just under 3000 psi and began shooting at 50 yards. I was pleasantly surprised as the first 8 shots formed a group 7/16" then all of a sudden the groups began to abruptly open up. I assumed that the tiny group was just luck and that this particular pellet was not the one, then when I recharged the rifle I was welcomed with another 8-10 shot one hole group followed by an abrupt loss of accuracy. I thought velocity drop might have had an impact on the downrange accuracy, so I shot a 25 shot string over my chronograph to check the velocity (I had never actually shot my Marauder over the chronograph) and discovered a rapidly dropping shot string. What I wanted to ask is why am I loosing accuracy with the loss of velocity and pressure? How should I solve this problem? I repeatedly recharged the rifle and observed the same results so I am sure that that the barrel is capable of good grouping, but something is changing as the pressure drops effecting accuracy. Thanks in advance for any help and tips.

Andrew           

[Ribbonstone]

Just wondering..would you characterize the bad groups to type?

 4-and-1
two separated clusters
strung out in a general direction (vertical/horizontal/ or some diagonal type of stringing)
large over all with no real pattern

[aack73]

maybe retune it for a better shot string curve. if you are getting great accuracy out of a particular pellet and velocity, tune the gun to shoot a nice curve around that velocity. if accuracy is king, take what/as many shots as it will give you at that  velocity. you could always squeeze a couple hundred psi more in it. and tune it accordingly. less than 1/2" at 50 is awesome.  

for more info for the rest of marauder owners, what pellet, velocity, and barrel you using?

[.22 Varmint]

Just wondering..would you characterize the bad groups to type?

When accuracy dropped off the shot's began to hit at almost complete random below the POI, the average bad group was about 2.5-3 inches, there was so real pattern in these groups such as stringing or separate clusters in one groups.

for more injavascript:void(0); for for the rest of marauder owners, what pellet, velocity, and barrel you using?

I was using Barracuda Match 21.14 grains with the 5.53mm head diameter, the velocity starts at the mid 700's but very rapidly drops from there, I think the gun can do better. The barrel is just a Crosman barrel with a crown job from Mac 1. The original barrel from in my Mrod from the factory was terrible and would not shoot any pellet accurately so this one is a replacement.

less than 1/2" at 50 is awesome.

Yes, It sure is, I just have to figure out how to make it shoot like that for 30 shots, or something near that.

Thanks for the replies,

Andrew

[rsterne]

I can't speak to the acccuracy issue, but if your shot string starts out high and then drops from there you have to back out on the hammer strike.... With the MRod this can be done by decreasing the hammer travel (ie clockwise on the screw).... decreasing the hammer spring preload (ie counter-clockwise on the screw).... or a combination of both.... If you are shooting primarily at 50 yards, I would aim for the first shot to be about 4% below the peak velocity and then stop shooting when the velocity again reaches that velocity.... If all the shots are within 4% of the maximum, then POI change at 50 yards should be only about 1/4"....

Once you achieve a nice smooth bell-curve, then try different pellets to see if you can get a reduction in group size....

Bob