To archive the sufficient recoil to operate a gun you will get lower preasure with heavy bullets than with lighter bullets.
This is like compereing 9x19 and .45 acp.
,45 acp have much easyer to operate an automatic firearm than a 9mm have and the recoil of the .45 is more but slower. And still the .45 acp is working on half the gaspreasure of the 9x19.
To archive as much recoil energy with the 9x19 as the .45 acp you have to load the 9x19 really hot( ie bloody high preasure).
I fully agree with you about the unneccery extra spring in the foreend of the carbine.
What the gun is needing is extra power in getting the action to open and not to close.
I think the only way of solwing a really heavy barrel on a luger is to have a gaskick betwen the barrel and the forend.
This way the heavy recoilspring could still be there.
Regards HÃ?Â¥kan
http://www.vapensmedjan.com