Slide Won't Cock Hammer: How to Make the Return Spring Weaker?

[lognom]

Hi,
I just replaced my return spring (on my FAS 6007) and now every other round the slide won't cock the hammer. I'm assuming the spring is too strong because I never had this problem before and everything else like ammo and my gun cleaning is the same. Unfortunately, I threw out the original spring that I mistakenly thought was defective. How can I make this spring weaker? Should I remove it from the gun and compress it? Cut off a turn?

Thanks,
Lloyd

[Gwhite]

Cutting off turns will work, but if you've only got the one spring, go slow.

What ammo are you using? Just because it's "standard velocity" doesn't mean they are anywhere near all the same. > 90% of regular rimfire ammo is sold for rifles, and is tested in a 16" barrel. The velocity you get out of a 5" pistol barrel can differ wildly, as will the recoil characteristics.

As an example, most Pardini SP's will shoot CCI standard velocity just fine. If you want Aguila standard velocity to work reliably, you have to cut the spring from 132 mm long down to 111 mm.

[lognom]

I'm using the same CCI sv that I was using before the spring change. Could I compress the spring by cocking the hammer and leaving the gun for an hour or even overnight? I'm assuming that this Chiappa spring has the same number of turns as the original but since I threw the original out I can't be sure.

I do have other backup springs both from Chiappa and from Wolf.

Thanks,
Lloyd

[BobGee]

Certainly leaving the pistol cocked over night would soften the spring a little. In fact Pardini recommend “seasoning” their new pistol springs by leaving the pistols cocked overnight and the magazines being left fully loaded overnight too (preferably NOT in the pistol).

Bob

[Gwhite]

Pistol springs are a whole mess all on their own... Manufacturers vary the designs over time (length, number of turns & wire diameter), and manufacturing tolerance for the wire can have a big effect. The stiffness of a spring varies as the 4th power of the diameter, so even a very small change can have a big impact on the forces the spring produces.

I have a spreadsheet of Benelli .22 recoil springs, spanning over 20 years of production. The number of turns vary from 47 to 61. The wire diameter goes from ~0.65 mm to ~0.72 mm. The change in wire diameter alone makes a 50% difference in the force produced when everything else is equal.

MatchGun MG2 Evo springs nominally come in 3 strengths, which they pick from at the factory for best functioning. I obtained spares in all three different strengths to experiment with. I don't know if they changed things or mislabeled them, but I have a pair of "normal" springs that are identical to a pair of the "soft" springs, and NONE of the three pistols I've examined came with springs that are as stiff as the "soft" springs...

If your pistol is failing to cock half the time, and nothing else has changed, then cutting the recoil spring down will probably fix it. However, all it takes is a bit of dirt or a tiny burr to add enough drag to make things mess up, so make doubly sure everything else is OK. Once you've found a spring length that works reliably in a clean pistol, you will need to take off a bit more to deal with dirt that accumulates from firing in a long match, plus a bit of extra margin. When I was coming up with the best length to shoot Aguila SV in Pardinis, I spent a while cutting off one turn at a time until I could go a reasonable number of rounds between cleanings.

Also, if it's just the recoil spring, things should be worse for the 2nd round. That is when the upward force on the slide is at a maximum from the remaining rounds in the magazine, which increases the drag on the slide. If the pistol fails to cock just as often after the 4th or 5th round, the problem may be elsewhere.

[lognom]

I'll clean the slide and frame again thoroughly before I mess with the spring, because the problem is just as bad after the fourth and fifth rounds. The reason I suspected the spring is because it never happened once before I changed the spring.

Thanks to you both in any case.

Lloyd

[Gwhite]

I don't know what's involved in changing the spring on the 6007, but if a burr from manufacturing was floating around inside, it may have landed someplace where it's now causing trouble.

Weird things happen. I've seen a couple pistols over the years that started jamming after the firing pin was replaced. In one case, when the old one broke, it left a tiny burr on the bolt face, which prevented rounds from sliding up into place. On the second, the firing pin shape was off and was causing excess drag.

It could still be the spring. For example, if it's slightly over/undersized.

[lognom]

I'll re-install the spring and make sure it's clean. I'll also try another backup spring too. Thanks.