The spring holds pressure on the firing pin safety, to hold it down. It could potentially be unsafe to shoot the gun without the spring. The firing pin safety blocks the firing pin from hitting the primer unless the trigger is pulled. With the spring missing, the firing pin might not be not blocked. You need to replace the spring before you shoot it. There is also a little flat cover that goes over the spring. Do you have that?
The springs are available now, the part number is 27749 and is the same for all the M&Ps, full size or compact. I did the same thing and modified a Glock spring that works. But only used this setup to check the new sights I put on. I will not carry the gun until I get the factotry springs. I ordered a few because eventaully I will be changing all the sights on my M&Ps. Anyway, call Smith and order a few. They are less than 2 dollars and it's probably a good spare part to have on hand.
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