Sunday, August 17, 2008

Accurizing a Marlin 780 .22 rifle

I’m working up to a test of .22 rim fire ammunition. Looking at both velocity variations and accuracy, I would like to fire each round from the same rifle through out the testing. Browsing the Carteach0 armory, only one rifle seems to fit this bill; My Marlin 780 bolt action fitted with a 4x scope.

Bought many years ago, the old Marlin had its wood stock refinished and the trigger modified slightly. Proving to be a consistently accurate small game rifle, it has accounted for untold numbers of squirrels and rabbits. Almost boringly easy to shoot from a bench, it has also been used to instruct several young shooters in safe handling and trigger control.
For this test, I’d like the ammunition to be on trial and not the rifle. To my way of thinking, that means I needed to go over the rifle and smooth any rough edges accumulated over the years. We all know what that means…. Off to the work bench we go!

The first step was to strip down the rifle and evaluate it. While it has been well treated, it certainly has been used. Things wear, bedding changes, and dirt accumulates.

The barreled action was dismounted from the stock, the bolt removed, and the scope dismounted. After a quick wipe down of any obvious grime, the barreled action was refitted to the stock and the bedding checked. The action portion was still fine but the barrel channel needed attention. The walnut stock had warped upwards slightly over the decades, and this was applying pressure to the barrel in an uneven fashion.

Snugging the stock into a cleaning fixture, I carefully removed wood from the channel using a specialty rasp made just for this job. Refitting and testing every few minutes, it took about thirty minutes to relieve the errant channel. I finished the job with some 600 grit paper wrapped around an aluminum shaft slightly under barrel diameter, smoothing the channel to almost a gloss finish.

Next, I turned to the trigger. Years ago I had fitted the trigger guard with an over-travel screw, and the trigger itself with new springs. Shims were installed between the trigger and its mounting to take away slop. At that time it was also polished on the contact points. Inspecting the trigger now revealed it to be in good shape, needing nothing more than a good cleaning. After some Q-tip cleaning action involving Ballistol, it was greased with Molybdenum lube.

From one end of the rifle to the other, now the muzzle came under study. Knowing that a proper crown is required for decent accuracy, many folks re-cut and shape their muzzles accordingly. Not being able to think of a good reason why this rifle should not have an eleven degree target crown, I proceeded to cut it so.

Using a crown cutting and chamfering tool kit from Brownells, I set up a cutting tool with an eleven degree cutter head and a .22 caliber pilot. Heavily lubed with cutting oil, I carefully made one turn at a time by hand, cleaning chips every rotation. In only ten minutes of work I had reshaped the roll crown into a sharp target crown exactly in line with the bore.

The Marlin Microgroove rifling does look unusual when viewed at a freshly crowned muzzle. I will likely re-cut a slight forty five degree indent to protect the fine rifling edges.

All significant changes being done, the barrel was given a good scrubbing with
Ballistol. Following this the bolt was field stripped and cleaned, and then the whole assembly was lightly lubed and reassembled.

Once assembled, I used my RCBS trigger gauge to check the trigger pull. A reliable two pounds, tested a dozen times. Surprisingly crisp for such a cheap rifle, it will more than suit for small game hunting and bench shooting.

The scope was reinstalled with fresh drops of loctite on the mounting screws. A few sighter shots at approximately sixty feet found each round widening a single whole in the metal burn barrel I was firing at, only an inch or so from point of aim. I’ll save final sighting for the bench at the range, where I’ll be running my rim fire ammunition testing. In fact, it might get sighted in today!

I'll let ya'll know how it turns out!

(Update with a warning.....)

Please be aware... just because I chose to hog out the barrel channel on this rifle and give it clearance does not mean every rifle shoots better that way. In fact, many rifles only shoot well with some upward pressure at the muzzle. I'm even aware of one military Mauser that will only shoot well with some downward pressure at the muzzle (I own the strange beast).

For this .22 Marlin, range time today demonstrated this rifle requires upward pressure to shoot well. I had to shim it with four thicknesses of card stock between the stock and barrel to get the pressure just right. Having done the experiment and figured this out, I'll now inlet a brass shim to apply that much pressure.

Once the bedding issue was diagnosed the rifle settled down and began shooting well. I was only sighting it in this afternoon, but it quickly started shooting quarter sized groups at fifty yards with cheap ammunition.

I have decided I'll need a higher magnification scope to do real accuracy testing at any kind of range. The four power generic Deerfield scope on it now has cross hairs that cover a one inch group at that range. Hard to really get fine accuracy that way...

Still, it is gratifying to see the old girl can still dance the dance



immagikman said...

Wow, some times it is surprising to see your self as a veritable novice after having thought you were fairly gun savvy :) Very instructive article.

Oh and hey, I just saw your post on my blog about the SLCFSA, it may in fact be too long a trip for me, but it is worth looking into. Im already looking at a 40 mile trip to a range, what's another 100 :)

Carteach0 said...

I updated this post, as I had to change something. It took some shimming to return stock pressure at the muzzle in order to make it settle down and shoot well.

Some rifles are like that.....
Something to be aware of.

Old NFO said...

Excellent post Carteach0! Are you going to try any Eley through it? I also envy the bench and tools you have to actually be able to work on your stuff! I wish I had that luxury...sigh...

Carteach0 said...

NFO..... Yes, I'll try Eley as well.
Slowly assembling a bunch of ammo, and a partner to help me that day.
Shall be enjoyable!

Old NFO said...

I'll be curious Carteach0- Especially after I priced some Eley the other day... DA*N that stuff has gotten pricey!!! Thankfully I've still got a couple of thousand rounds if I ever get the .22 pistol out.

immagikman said...

Looking forward to the the results. Maybe some this weekend?

studentofthegun said...

Mr. Carteach0
How can you tell a barrel needs a shim? What are the tells?