The weak spot of the 70pps is the barrel union!
The receiver is designed for the marlin 60 or similar guns with larger diameter barrels.
A smaller diameter barrel is needed to allow the securing nut to work on the machined shoulder.
Marlin skimped on design cost and tooling by using the standard receiver and creating an adapter to fill the gap.
This receiver is secured by a cotter pin type arrangement thats driven thru the receiver & presses hard against a rebate in the barrel adaptor, this stops all movement in theory but i have found that any extreme over-tightening of the retaining nut puts excessive pressure on the barrel adaptor securing pin. Once the pin is damaged, then the whole barrel can easily move between shoots-especially if your shooting heavy hitting stuff like velocitor's or yellow jackets or CCI stingers!! Not only is this problem evident while tightening the barrel down but it is also evident when attaching and removing my silencer!
It seems that marlin barrel security has never been subject to million of silencers being tightened and slackened again and again in USA, however in Europe silencers are totally unprohibited in most countries
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Anyway besides the problems with the barrel adaptor there also exist the lesser issue of slight movement of the actual barrel within the barrel adaptor. I've thought about this problem quite a bit and there are solution but they'd require the design and machining of a new adaptor!!
Moral of the story! Dont overtighteneing your barrel and fit a scout scope!!
Although this problem happened to me- I sorted it by taking the adaptor out and filling in the rebated with a fillit of weld and then filing it down to the original size before re-driving in the securing pin. I had to do it 3 time to get it right--
Originally i tried sorting the issue by using fiber glass resin (resin only). I poured a small bit in to the securing pin hole before inserting the pin. This worked for a while but it fail while firing more hard hitting ammo!!.
TBH it would seem that a few grub screws placed at the thickest points of the reciever housing and screwed in to the barrel adaptor would stop any movement of this 5th wheel.
regards ivan