I have this old marlin 60....

[Aliquippa]

Finally got around to shooting Dad's old marlin again that he left me. I tried shooting an AQT at 50 yds. After I got it zeroed in (benchrest) I took a shot at dead center of targets and it was more or less spot on shot from bench. Incredibly accurate. Then...I tried shooting prone and wth? My shots though grouped were like 2inches lower. I adjusted moa upwards and then they were too high. So my question is:
Could the pressure of sling on the rifle make it shoot wonky like an M1 when using a sling. The sling attachment slides over the barrel and tube feed. Or
could the scope itself be wonky (possibility due to age and being low end Bushnell) or maybe I'm just a hoser and should practice with it more or I should just try with a different scope?

[Sim]

Please take my thoughts with a grain of salt, I do not have nearly enough experience to be an authority here.

One thought is that the recoil of the rifle may be different because of the different amounts of contact with the rifle stock/sling/barrel in different positions. You could maybe try to rule that out by shooting from a bag or other support prone instead of using your sling, and seeing if the POI matches your supported shots from before.

Another is that the scope may be set to correct parallax at a distance that is not 50 yards, and the change in your head position relative to the scope is manifesting that parallax error. I don't know how to test for that other than checking the parallax on your scope...

[Leadslinger66]

Finally got around to shooting Dad's old marlin again that he left me. I tried shooting an AQT at 50 yds. After I got it zeroed in (benchrest) I took a shot at dead center of targets and it was more or less spot on shot from bench. Incredibly accurate. Then...I tried shooting prone and wth? My shots though grouped were like 2inches lower. I adjusted moa upwards and then they were too high. So my question is:
Could the pressure of sling on the rifle make it shoot wonky like an M1 when using a sling. The sling attachment slides over the barrel and tube feed. Or
could the scope itself be wonky (possibility due to age and being low end Bushnell) or maybe I'm just a hoser and should practice with it more or I should just try with a different scope?

I think you are most likely correct. Sling pressure on the barrel will change your point of impact. Which direction and how much really just depends on the angle of the sling and the amount of tension you use. Using the exact same sling tension every time would help, but that is difficult to do. If you are willing to drill the stock, you could add a sling mount underneath to help.

The scope might have issues, but I would eliminate the sling variation first.

[Bundt Pan]

There was/is an old post somewhere where this phenomenon was looked at in a more scientific way, but what i recall was a consistent shift in POI from bench to a slung position. I should have bookmarked because i don't recall how exactly i searched and found it.

[.416_Rigby]

100% sling tension will move your POI when the sling is attached to the barrel. Even a sling mounted to a stock with a non free floated barrel can noticeably affect POI. I don't do engineering math much anymore but I'll chug some caffeine and run through it to prove the point.

What we have is a cantilever beam problem. Let's assume the barrel is rigidly mounted to the receiver and the sling is applying force to the barrel 12 in aka 1 ft down the barrel. I'll neglect the stiffness of the stock if the barrel touches it (we have Kentucky windage, is this Kentucky engineering?). Let's say the sling is pulling straight down with 10 lbs of force. Barrel stiffness comparable to stainless with a modulus of 180 GPa, barrel is .75" in diameter with a .22" bore. Need to calculate the area moment of inertia I = π(do^4 - di^4) / 64 where do = barrel diameter, di = bore diameter. Plug into a solver and we find .01431" of deflection at the point where the sling is pulling on the barrel. Similar triangle that out from 1 ft to 150 ft and we're looking at ~2.15 inches low at 50 yds. Sweeeeet, the math maths.

You have options to deal with this, and Marlin 60's are tackdrivers so it could be a fun challenge to overcome so you get that sweet sweet accuracy edge over 10/22s. As Leadslinger said, mount sling to stock if possible so you don't pull on the barrel (if not free floated this may not totally solve your problem). You could use less sling tension - half the force half the deflection. You could also move the sling closer to the action, a couple inches will make a big difference. And of course you can use the same sling tension and angle every time and remember to hold under when standing or grab clicks between prone and standing.

Here a CMP shooter explains the same effect when moving from prone to standing with a non free floated barrel
https://youtu.be/HNBpGUri2mo?si=23Fj6Smr2D02YcBR&t=490

[Dmthomp32]

Switch to sling swivels on the base of the stock. Loop pressure on a barrel will 100% cause a shift in impact. I was planning to use my Sig 522 in an Appleseed shoot, it's very reliable and very accurate! However when I slung up and practiced, I watched my point of impact drop .5" at first in stage 1 and all the way to 2-3" by stage 4. Very real change of impact and made my use of the rifle pointless.

[Mrs. Smith]

My 25m rifle of choice is a Marlin Model 60 (don't you roll your eyes at me, M.O!  )

I went ahead and mounted a sling point in the stock to minimize that effect.  It moved way too much when using the barrel-mounted sling point. No bueno. That helped a bunch.

Eventually I mounted it in a Boyd's stock and that eliminated the effect entirely.

Also, I zero any rifle using the techniques I'm going to do shoot with. In this case, I zero prone in a sling, regardless of rifle platform.  I zero my LOANERS from a bench, virtually guaranteeing that the student borrowing it will need to perform IMC

[dond]

Even ignoring the sling tension on a barrel, there is still a problem even with free-floated barrels. It is treated on page 72 of the IM under the topic of Time Lag: Recoil. When a rifle fires, it recoils backwards, and the  point of contact of the stock on the shoulder is generally lower than that of the barrel. Therefore, the rifle will rotate upwards about that point. The rotation will be damped by anything touching the rifle, such as your cheek, the support hand, and especially the sling tension. When firing my AR using a bipod, and then with a sling in the prone position, the POI with the sling is about 2 MOA lower than with a bipod.

[The Wolfhound]

Even ignoring the sling tension on a barrel, there is still a problem even with free-floated barrels. It is treated on page 72 of the IM under the topic of Time Lag: Recoil. When a rifle fires, it recoils backwards, and the  point of contact of the stock on the shoulder is generally lower than that of the barrel. Therefore, the rifle will rotate upwards about that point. The rotation will be damped by anything touching the rifle, such as your cheek, the support hand, and especially the sling tension. When firing my AR using a bipod, and then with a sling in the prone position, the POI with the sling is about 2 MOA lower than with a bipod.

Oh yeah!  Been there and wondered WTF?