Attempted to replace a lost Model 4506 recoil spring with a heavy duty one.
1. the heavy duty recoil spring is 47 coils and two inches longer than the OEM spring.

2. finally got the replacement spring installed after over coming the kinks in the spring from trying to push it into the recoil spring housing.

2. discovered that the recoil spring guide rod will not stay in the half moon cut out in the barrel lug due to the more powerful spring.

3. the half moon cut out in the barrel lug appears to be too shallow.

I attempted the same procedure with a Model 645 with the same results.

I recall there being two different lengths of recoil spring. One for hardball and one for lighter target loads. Using the wrong one is not advised. Somebody else will come along with a better answer.

I'd try to find a duplicate of the original spring before trying anything else. Extra power recoil springs are frequently an answer to a problem that doesn't exist.

Two inches longer than OEM strongly suggests someone sent you the wrong spring. Couple of coils is one thing, two inches is just WRONG!

S&W part numbers as follows:

Recoil spring-wadcutter: 200710000
Recoil spring-standard: 201610000
Recoil spring set (both): 190320000
Recoil spring guide: 103200000

Always stay with factory weight springs. The engineers who designed the gun know more than we do about it.

The factory S&W standard weight (p/n: 201610000) spring is 38 coils 0.044" wire. A new spring prior to installing measures about 8.125" long at rest.

If you have 47 coils then I guess someone sent you the wrong spring.

Best practice to stick with the factory weight springs so long as they are available. Extra power spring don't solve anything and do create other problems.

Edited to add... Midway has stock Smith & Wesson Recoil Spring Standard S&W 1006 1026 4505 4506 4526

Cheers
Bill

__________________
Because they don't make a .46

The OP is using the factory spring set - S&W Model 4506

The fact this is a used gun makes things more difficult.

I bought a 4506-1 NIB years ago and it was a royal pain to reinstall the recoil spring, rack the slide and in general manipulate the gun.

Some jack@#$ told several guys that using the short target weight spring for 300-400 rounds would help the gun break in. Guys did that and several ended up cracking their frames.

This could be a case where the previous owner used the short spring and the OP is comparing a full weight spring to a worn out target spring.

Edit to add I just went and checked and my spring, the DUTY/Hardball Spring, is 38 coils