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lampy · Veteran Member

Let the fun begin Smile

I got the 766 shooting as a pumper, I wanted to do this so I would have a base line to compare to after the conversion. I'm starting with a Phase 1, Crosman 766 made between 1975 to 77. The barrel is the same soda straw found in the later version 766 and the 2100, with one difference the barrel is encapsulated in plastic.

The valve is a bit different, it is integrated into the breech. I like this since it eliminates the need for a transfer port seal. One less place for a leak.

This 766 is shooting CPHPs at about 580 fps at 10 pumps that is 50 fps below advertised fps of 630 fps. CPHP and RWS Meisterkugeln are not accurate pellets in this rifle. I'll have to shoot some more pellets to see if I can find one it likes.

Stay tuned, this might take a while. I just ordered parts for this project from Crosman today so they are a week out.

Slavia · Moderator

Staying tuned. Same bat time, same bat channel.

lampy · Veteran Member

While shooting as a pumper I kept noticing an occasional flier, one out of ten shots would land well outside the main group. I did some checking and found the pellets were being damaged sliding into the breech. I cleaned up the breech with a D- bit I had made for a previous project. This got rid of the random fliers but didn't really improve the accuracy that much. The three groups on the left were shot prior to the breech repair with the three groups on the right were shot after.

Now that it is shooting as well as it can I can get on with the conversion. The 3/4" DOM that will become the reservoir.

The slots are milled.

It has been a bit chilly this weekend, with temps around naught..... EyesWideOpen

It's a bit warmer in the garage but not much.

lampy · Veteran Member

My parts arrived from Crosman. I'm not 100% sure I'll end up using the gauge but it would be handy for running this on HPA. I'll do some testing first and depending on how things turn out will decide if the gauge gets installed.

lampy · Veteran Member

The plug is made for the fill fitting. I was going to use roll pins to hold it in place but I like Big Bore Barts idea of using socket head cap screws to hold it in place.

lampy · Veteran Member

The fill plug attached using socket head cap screws. Thanks for the idea Bart!

The valve is normally held in place by the two halves of the receiver, I added a couple button head cap screws for extra insurance.

Assembled and the first shots fired on CO2.

Nothing really impressive to report yet. The first string over the Chrony showed the first 3 or 4 shots at around 570 fps, which is about the same as I was getting at 10 pumps. The fps dropped steadily after that until I hit 500 fps at shot 14. I was hoping for more shots per fill on CO2.....
I have a couple thoughts in mind, disable the check valve to see what effect that has and I have room to add another 6 inches of reservoir to get more shots per fill.

Robw · Veteran Member

"The valve is a bit different, it is integrated into the breech. I like this since it eliminates the need for a transfer port seal. One less place for a leak.

"

So the compression chamber "valve" is attached directly to to barrel?
Could you post a photo of it attached to the barrel as I would like to
see if I could adapt the same technique to my 1322 to eliminate the
funky transfer port and seal.

lampy · Veteran Member

Robw, next time I have it apart I'll get some more detailed pics of how it goes together.

lampy · Veteran Member

I was less than happy with the shot count on my first tests. I made a couple changes. I changed the hammer spring to a lighter and shorter spring. While I had it apart I also drilled the center out of the check valve.

These changes made all the difference. I saw an increase in velocity, Now I'm getting a little over 600 fps. I assume the increase in velocity is from drilling out the check valve. The lighter hammer spring increased the shot count dramatically. It took 25 shots to drop the velocity from 610 down to 580. It took another 17 shots to drop the velocity down to 500 fps. Giving me a total of 40 usable shots, quite an improvement over 14 shots I got on the first tests.

The CPHPs seem to like the higher velocities also, with the first 25 shots going into a dime sized group.

lampy · Veteran Member

It was HPA tests today. Things look good. I was expecting a low shot count on HPA but was pleasantly surprised.
For the first session I pumped it up to 1000 psi. I fired 12 shots over the Chrony. The FPS dropped steadily from 678 on the first shot to a low of 586 on shot 12.
The second session was at 1500 PSI (it took 40 strokes on the Disco pump)
The first shots were at 730 fps dropping to 710 fps at shot 10. From shot 10 velocity dropped slowly down to 623 fps by shot 22, After shot 22 fps dropped off quickly.

This is a pic of the first HPA test target, shot at 10 yards with CPHP. The top target is the 1000 psi fill and 12 shots. The lower group is the 1500 psi fill and 26 shots.

This pic is of targets shot on a 1500 psi fill using CPFT (cardboard box) 10 shots at each target. This shows what the numbers don't, this setup would be a great short range plinker. 36 of the 40 shots went into a dime sized group.

Over all I'm pretty happy with the way the 766 shoots on both CO2 and HPA.

CPO · Member

Looks like I can call crosman about a pressure gauge for my Mrod. They have been telling me since December that they are out of stock. Same gauge and fitting as in your picture!

lampy · Veteran Member

lampy · Veteran Member

I just wanted see if I could get this chart to upload. It makes it easier for me to visualize how things are working with a chart like this. Looks Like it wants a higher fill pressure. I think I'll try an 1800 psi fill and see what it looks like.

lampy · Veteran Member

I tried the 1800 psi fill and got 20 shots above 700 fps, double the number at a 1500 psi fill. I don't really feel comfortable pumping this thing up that high. I had a lot more luck with accuracy using the cardboard box CPFT pellets so I went back to CO2 to see what the shot groups looked like.
30 shots into a dime sized group! I was shooting over the Chrony and the velocity had dropped from 600 fps down to 570 at shot 30. I finished off with 10 more shots at the lower target and the group started opening up as I expected.

I'm pretty much done with this project so I thought I'd put the front stock on.

The HPA test were interesting but I like the way it shoots on CO2 so I think it will spend it's life shooting on CO2.