Air Arms MPR Power Upgrade

By Dave Franey

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Disclaimer: this website takes no responsibility for the instructions detailed in the article below or the results of following these instructions. All airgun work should be performed by a qualified airgunsmith. If the reader decides to attempt repairs/modifications to ther airguns, they do so at their own risk and this website and author take no respsonsibility for the results of such actions.

Credit for this idea goes to poster RFC. He advised me on the dimensions and method of disassembly.

Step 1: First be sure rifle is unloaded. Remove the air cylinder from the action and then remove the action from the stock. Now use a #10 Security Torx bit to remove the power adjustment cap screw as shown in Below

Step 2: Now remove the power adjustment grub screw with a 1.5mm allen wrench and then put the power adjustment cap torx screw back in.

Step 3: These 6 screws need to come out and the bbl/upr receiver lifts up/off to get access to the hammer spring.

Step 4: Be careful with this tiny o-ring when you lift the barrel & upper receiver off the lower receiver.

Step 5: Take out the rear receiver screw with a 5mm allen wrench. BE SURE TO HOLD BOTH PIECES as you remove it as the hammer spring will force the two peices apart.

Step 6: This is what the stock 7 Joule hammer and spring look like before the power upgrade.

Step 7: This is the spacer. It is made from 1/2" copper tubing cut to 0.260" in length and deburred on both ends. The 1/2" copper tubing was cut with a tubing cutter and a smoothed with a file.

Step 8: Slide the "0.260 copper spacer onto the stock 7 Joule hammer and then slide the hammer spring on behind the spacer.

Step 9: Now reassemble the action in reverse order. Do not re-install the power adjustment grub screw removed in step 2.

I did a 115 shot test with CPL starting at 200 bar down to 100bar. I'll just give you the highlights skipping by tens. BAR is estimated by reading the gauge on the air cylinder, very tough to be precise.

Shot#	FPS	BAR
1	730	200bar
10	728
20	752	180bar
30	784	165bar
40	786	160bar
50	791	150bar
60	792	145bar
70	808	140bar
80	806	135bar
90	802	120bar
100	806	110bar
110	759
115	757	100bar

So far looks like a 160bar fill gets 60-65 shots. I also did about 10 shots with JSB heavies at 160bar, average around 745 for 12.6 fpe. Basically zero cost to go from 6fpe to 12, pretty cool. Thanks RFC.

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