Rechambering Valmet m88 to accept 5.56?

the problem you might encounter is rate of twist in the bore. slow rate of twist 1 in9 or more will prefer 52gr projectiles or less, faster rates of twist like 1 in7 will prefer heavy projectiles like 60gr. most 5.56 is 55gr or more.
 
I thought the same thing when I first bought it, ran a box of PMC 5.56 and it was not liking it at all. Had problems like high recoil, bullet torn out of the case, jamming but when I switched to .223 it ran smoothly. The problem was that its chambered specifically for .223 so when I was running 5.56 though it it would create a false headspace and would fire at a way higher pressure. Atleast thats what I've gathered from looking around online
 
Is the bore chromed? If it is, reaming could be problematic.
Might be easier to find a gunsmith with a Wylde chamber, rather than a 5.56x45 one.

5.56x45 ball is basically available with two bullet weights - 55gr and 62gr. The latter is the "greentips" version.
The only reason that some 5.56x45 rifles have 1:7 twist barrels is to stabilize the long tracer bullet.
What is the twist of your .223 barrel? 1:12? 1:10? Either will shoot 55gr ball.

Notwithstanding the SAAMI warning about shooting 5.56x45 ammunition in .223 rifles, yours is the first one I have heard of exhibiting the symptoms you have reported.
 
I thought the same thing when I first bought it, ran a box of PMC 5.56 and it was not liking it at all. Had problems like high recoil, bullet torn out of the case, jamming but when I switched to .223 it ran smoothly. The problem was that its chambered specifically for .223 so when I was running 5.56 though it it would create a false headspace and would fire at a way higher pressure. Atleast thats what I've gathered from looking around online

WTF does this even mean? The two cases are identical. Headspace is governed by the case.
 
Issue found, stock recoil spring wasnt strong enough to handle the extra pressure of 5.56. Swapped to a galil spring and it runs fine in both cals
 
Issue found, stock recoil spring wasnt strong enough to handle the extra pressure of 5.56. Swapped to a galil spring and it runs fine in both cals

Had the same issue, and fixed it the same way, but with a Wolff extra power AK recoil spring. Also, I’d highly recommend getting a KNS adjustable gas piston, they make one specifically for the Valmet.
 
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