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