I bought CP chrome silicon springs for all my 20rd mags, installed them in Colts and Adventurelines all straight not curved.
They went from 20rds to 18 & 19rounders, would not strip first rd or insert into a closed bolt gun if you could get the 20rds in.
So after closer inspection.
Wire diameter is .003 larger and have 1 more coil than originals.
Clipped 1 coil and can now get 20 in but are tight.
Spring rate way better higher than originals.
Are these springs for curved mags or is this normal on a 20rd. spring replacement?
Going to try them like this for a while and trim 1/2 coils until they work right.
Anybody else been down this road?

May I suggest you contact Larry from CP.  His username is tochhc235.  He's a stand up guy and will definitely get back to you.

Hope this helps.

Justin

Our curved 20 round magazine holds 20 rounds without a problem using a Generation II Magpul follower or standard green follower. If you get a minute give me a call 866 274-0247 and let's talk about it and see if we can come up with a solution. Thanks

Larry

Let me first apologize for the lousy picture, but at 0330 this is what I have to work with.  
Starting from the top:  USGI 20 round spring, 20 round curved spring from a Brownell's mag, 30 round USGI spring.  

As you can see, the 20 round curved spring is about 1.5" longer.  There are two more coils and my caliper showed it was about .008 larger in diameter (same as the USGI 30's).  The longer springs make sense, since the mag bodies are about half an inch longer.  

I don't think the springs for curved 20 round mags will interchange well with the 20 round USGI springs.

The 20 rd CP springs are bigger gauge wire.
The CP 20rd springs are 1 coil longer and designed for CURVED 20s.
To use in STRAIGHT 20s a min of 1 coil needs to be cut off or you have coil bind before you get 20 rds in. This is with stone stock alloy followers in Colt And Adventureline mags.
Also the first coil under the follower  on a CP spring is a full wind giving you 1 more wire thickness in your coil stack on the front (short) side of a straight 20, where a factory 20 spring is not, the first coil is bent at a 90 and then looped to hook onto the follower not adding to stack hieght.
That being said I cut 1 coil off mine and can get 20rds in but they are tight, I am going to shoot them and see how they work.
If it slows my bolt to much on a closed bolt reload I will start with cutting another 1/2 coil off, looks like they should be good to go, better spring rate, shooting will tell.

Larry at CP and I went over all the above and below.

And as a side note Magpuls followers will work in curved 20s.
You can mod a Magpul to put in a straight 20 but not worth the trouble, too much material is removed on the legs  to get it in and defeats the exellent design they got in the first place.
IMHO.

update;
Final mod, clipped another 1/2 coil for reliable operation on a closed bolt reload.
Total of 1.5 coils clipped on a CP spring to work in a straight 20. as always YMMV