Mini-14 Timing/Port Pressures

[def4pos8]

I'll be contacting Accuracy Systems tomorrow to purchase another of their variable gas port devices for my latest Mini, an NRA Special.

As with my stainless Ranch Rifle, the initial accuracy was only "minute of man". This was disappointing as I used a recipe that does roughly 1.5 MOA in the stainless Ranch. Five-shot strings essentially "shotgunned" about a 9-inch diameter circle at 100 yards.

A high-power fanatic was kind enough to remind me of something that I had read before in accounts of Garand's efforts with the M-1, various folks with the M-14 and Stoner's struggle with the AR-10/AR-15/M-16.

He said that I had to be certain that the op rod didn't move before the bullet had cleared the barrel. If the op rod moves with the bullet in the barrel, what I observed would be the result. When asked, I told him that I used a couple of ball powders. He said that the Mini-14 system was likely set up with extruded powders. Pressure curves, extruded versus ball, vary somewhat, with extruded tending to peak earlier and more abruptly; ball slightly more progressive and of slightly longer duration.

Varmint shooters with 24-inch bolt guns could care less. The problem for ALL gas rifles, AR-15 included, is that since ball powders will tend to put a bit more pressure at the gas port, a shooter/reloader has to be satisfied with a light to medium charge weight if accuracy is not acceptable with a full-power charge of ball powder.

Alternately, a means of moderating system gas pressure must be found. Some resort to high-energy recoil springs or special recoil buffers. Others use reduced-diameter gas port bushings. For the Mini-14, I have settled on the Accuracy Systems variable gas block.

Engineers in the '50s used the White-patent, sliding piston, gas cut-off for the M-14. That's why it's SO much easier to develop match loads for my M-1As. High port pressures are handled by the piston/cut-off.

Bottom line: control a Mini's op rod with proper pressures and its accuracy is equal to any other carbine. Does my fanatic friend's advice fit?

 

[ThomasT]

Its an interesting theory. I would think that the bullet is long gone before the operating rod ever started to move.

I think the main reason people add the smaller port bushings is to slow down the rod and take some of the force out of the gas system. The Mini has a pretty voilent system that can be tamed and still operate the gun reliably. Plus it helps to keep your brass in the same zip code you are in.

 

[ThomasT]

I guess i am the only one who is going to reply to your post.

I just ordered the gas bushings for my new as of yet unfired Mini-14 from ASI. What load did you develope for your ranch rifle? I have the standard 580 series with the straight barrel. Its stainless and wood with the new style sights.

Are you saying that your NRA special only shoots 9" groups with the heavier barrel?

 

[def4pos8]

My fanatic's ideas fit my results, unless someone comes up with better clues. Ball powders build pressure a bit slower than extruded but the pressure "peak" is more like "the top of the hill" -- properly high but with longer duration.

It's that longer duration that can result in high port pressures and consequent abrupt movement of the op rod. That op rod movement scatters the shots.

I've already tamed/"accurized" one Mini using the variable gas block. I suppose other folks might work the problem another way. I just wonder if anyone out there knows enough to confirm or deny our reasoning.


As an aside, my Fanatic Friend is an AR-15 shooter who hasn't missed the Camp Perry matches for many years.

 

[ThomasT]

I wish i could help you on the op rod causing accuracy problems. I still think the bullet should be long gone before the rod ever moves.

The Speer loading manual #11 used a Mini-14 for 223 data. They show a velocity of around 2900-3000 fps with most loads. I would guess that by the time the bullet makes it to the gas port hole it has to be going at least 2500 fps and maybe more. There is only 8" of barrel left before the bullet is free of the gun. I think barrel whip would be a more likely source of inaccuracy.

Go to YouTube and watch some of the videos that have slow motion shots showing the barrel and scope movement from the guns recoil. There is a good video of the AK-47 and AR-15 and the AK really whips the barrel when it fires.

O.K. here is the link. http://www.youtube.com/watch?v=G6BpI3xD6h0

Let me know what you think.

 

[def4pos8]

Oh well, I guess it's just us. I was hoping some stealth Ruger engineer might provide something!

I use Winchester or Federal cases for my best rounds. The primer pockets are "uniformed" -- reamed to SAAMI-spec depth. CCI #41 mil-spec primers provide the spark. I use Hornady "NewDimension", small-base sizing dies in a Forster press. All are trimmed to minimum case length. Powder charge is 23.5 grains of Accurate 2230. Bullet is a Nosler 69 grain. Hornady or Sierra 68/69 grain bullets work as well. Cartridge overall length is limited by the magazine to 2.240 inches.

This is a strong, but not silly, load. I worked it up in the stainless Ranch. It works.

I watched the video. It's interesting but doesn't address my Fanatic Friend's main point: if port pressure is too high, the op rod, even in an AK, (or AR bolt group) will start to move before the bullet clears the bore. Group size suffers. The poor AK in the video might be a victim of its ammunition! He says that high-speed photography of the forward barrel/muzzle must be used to directly observe all of this. He claims that the M-80 cartridge's bullet is nine inches from the muzzle/five inches clear of the flash hider before the M-14's op rod starts to move!

I'll do my aftermarket tweaks and enjoy the result. I'd just like some confirmation as to why!

 

[def4pos8]

And Another Thing!

Yes, my NRA Special, at its first session, only gave me about a nine-inch group at 100 yards with MY ammunition.

Of course, Ruger's lawyers would disown me in a millisecond because I built the cartridges. My earlier reply had the recipe.

The "heavy barrel" isn't all that much more heavy. The barrel does carry the chamber diameter a bit further and there is a "flare" just forward of the gas block but overall diameter, especially in the critical area of the gas block, remains at .625 inches. Shorter overall length versus the greater chamber mass results in nearly identical overall mass as my stainless Ranch.

I had some hopes. . . .

I'm not worried. I know that the adjustable gas block will permit me to tune the gas system and improve accuracy. As a Ruger stockholder, I'm sad that my company can't provide a product that will shoot well without forcing me to become an amateur ballistics engineer.

 

[benEzra]

The high-pressure gases pressurize the gas block and gas piston area the instant the bullet passes the gas port, while the bullet is still in the barrel. This force (hundreds of pounds) immediately begins accelerating the bolt carrier rearward. Since that pressure pulse ends as soon as the bullet exits the barrel, the gas piston has to already be moving at its full rearward velocity by the time the bullet leaves the barrel, though the distance traveled in that time is probably a couple of millimeters, or less.

The big problem may not be the motion of the gas piston, but the bending of the barrel itself that accompanies the rearward launch of the heavy gas piston/bolt carrier from the gas block. Don't forget that the mini's barrel is cantilevered off the gas block. When the gas block/gas piston is pressurized, one result is a VERY strong forward push on the bottom of the gas block (hundreds of pounds), resulting in a strong barrel bending moment with a very sharp onset. The resulting waves and vibrations in the barrel, while the bullet is still in there, can play heck with your accuracy if the vibrations are of sufficient amplitude and/or inconsistent.

Mini-14 gas systems are generally vastly overpowered, and I think one reason that smaller gas ports help with accuracy is that they reduce the barrel bending moments by softening the piston/bolt carrier launch. Switching to a powder that reduced down-barrel pressures at the gas port would have a similar effect, I would think.

 

[ThomasT]

Hello Benezra glad you jumped in. You may be right. Maybe the piston is moving quicker than i gave it credit for.

I did order the reduced size bushings today and plan on giving them a try. I will shoot my new mini with the factory bushing first before i switch out with the smaller size.

I hate to hear that the ball powder may be a problem. I just loaded 300 rounds with Ramshot Tac. A powder recommended by John Barsness for 223s that supposedly has a copper reducer added to it. Its a ggod thing i have two more 223s.

If those don't work i will try the 2230 load. At one time Natzches Shooters Supply had that powder for sale real cheap even though the exact amount escapes me. I thought about buying it for 30-30 loads as i didn't have a 223 at the time.

I hate to hear the NRA Mini din't shoot any better. I guess if i'm totaly disatisfied with my new mini i can sell it. They seem to sell well even if they don't shoot. A 9" group!!! Yuk.

 

[ThomasT]

He claims that the M-80 cartridge's bullet is nine inches from the muzzle/five inches clear of the flash hider before the M-14's op rod starts to move!

I forgot that part in the other post and i didn't catch it on the video.

That was my initial thought though. I have a hard time beleiving that the mass of the op rod can start to move before a bullet makes it out of the barrel. Maybe so but still hard to beleive.

 

[ThomasT]

I went back and rewatched the video. At the beginning when they are shooting both guns and showing the barrel flex they don't show the end of the M-16 but if you watch you can see that it too flexes when fired.

When they are shooting the wood blocks and show the slo-mo of the AK and the bullet exiting the barrel, the bullet is out the end of the barrel before the barrel has flexed or moved in any way.

So in the end, i don't know.

 

[benEzra]

He claims that the M-80 cartridge's bullet is nine inches from the muzzle/five inches clear of the flash hider before the M-14's op rod starts to move!

Again, the pressure pulse that causes the movement ends as soon as the bullet exits the barrel and allows the barrel pressure to equalize with the atmosphere. It may be that the amount of movement prior to pressure equalization is imperceptible on film (a mm or two), but all of the acceleration of the moving parts most definitely occurs before the barrel pressure is vented.

 

[ThomasT]

Well BenEzra again i defer to you. Like i said i'm not certain.

I did do a search here and the consensus seems to be that a 1-9 twist will handle a 69gr bullet which is what the OP said he was using. But that seems to be about as heavy as it will stabilize. I think i would try some lighter bullets just for grins.

The 300 hundred rounds i loaded were with 55gr bullets. I hope like the devil they shoot better than a 9" group. A 3 to 4" group at 100 yds is acceptable for what i want a Mini-14 for.

I forgot to mention it earlier but did you notice the group that shooter got at 200yds with the M-16. I wasn't impressed at all. My 30-30 shoots way better than that at that range. Maybe it was bad lighting. I don't like indoor ranges for that reason.

 

[Mike 56]

I changed my gas port bushing to .45 and it tightened my groups my about a 1/2 inch but i always thought it was the gas block being retoqured i never thought it was the bushing. My Mini would cycle would the smaller bushing i might give it a try. As for powder IMR 4064 has worked the best for me. My load is 24GR IMR4064, Hornady 52GR BTHP, C.O.L. 2.230, CCI400 primers, crimped with a Lee factory crimp die. With this load and improvements i have made i can shoot sub MOA at 100yd if i do my part. I made buffers for my Mini out of vacuum cleaner belts they did not make my Mini more accurate just nicer to shoot. Here are the improvements i made to my Mini with the most important listed first.

1 Muzzle Break
2 Trigger Job
3 Barrel Strut
4 Gas Port Bushing
5 Buffers

Mike

 

[ThomasT]

Hey Mike56 glad you joined in. I made the buffers like you posted on the other thread. I used some 1/8" hard rubber i had here. I also found the the rubber plugs at Home Depot for the Accu-strut. I haven't ordered the clamps yet.

I did look at the muzzle brakes. I hope i don't need one. I don't want my gun to be any longer than it is. I may have to though. I remember someone said they really improved there Minis shooting by adding a plastic bipod from CTD to the barrel. Something that dampens the barrel whip might be the ultimate solution.

Thanks for the load data. I like it when other people post what has worked for them. I plan on trying some of the 2230 and H4895 i have been stocking up on. I have some 4064 already. I use it in my 8mm Mauser loads.

 

[def4pos8]

Ahaaa!

As I expected!! Thank you for your comments, Mike 56, but you "cheated" by using an extruded powder!

4064 is a useful, flexible, middle-burn-rate powder that I have used successfully in 7.62x51, 7.62x63 and .375 Hollands' Magnum. My M-1A National Match loves the stuff.

My problem with the Mini-14 begins when I try to use those new-fangled, "modern", ball powders. Ball powders meter so well that I prefer them, especially when loading 300 5.56x45s instead of 40 .375s.

So it goes. . . .

 

[Mike 56]

Ratshooter, How do you like the buffers? I tried taking my muzzle break off after i put the strut on it opened my groups two inches so i put it back on. Who knows maybe yours will be fine with out one. let us know how the bi-pod works out. A Mini is a strange beast you never know what is going to make them shoot straight.

Def4pof8,You sound like you know a lot more about reloading than i do. My Mini load is just dumb luck i bought a set of Lee 223 dies on sale along with a box of bullets on sale from Midway. When i got my dies the only powder i had was IMR4064 and H110 so 4064 it was. If you ask me 4064 is an odd powder for 223 but did find it in one of my manuals. Infact if i remember right 24GR is the starting load.

The not so perfect Lee Perfect Power Measure does a pretty good job throwing 4064. Ever since i took it a part and wiped the inside down with a dryer sheet it has been working good and has that great April fresh sent.

 

[ThomasT]

Mike56 i shot my mini for the first time this afternoon. It was shooting low so 2 turns on the rear peep and i was hitting.

We were shooting at about 90 to 95 yards offhand at the far side of my buddies pond. I was using the cheap monarch ammo from academy. No paper was shot but it looked like i was hitting in a cantalope sized area. I was shooting at one rock and my buddy said every round appeared to be going in the same spot. No shots from a rest were taken.

I haven't done anything yet but add the buffers and retorqued the gas bushing. It was gapped on one side and touching on the other. The buffers worked just great by the way. No bipods or muzzle brakes have been added.

Needless to say i am very happy with my new gun. I hope that soon i can make it out to shoot at paper with some of my loads and fine tune the sights.

 

[def4pos8]

Well, it looks as though you guys are managing things all right. I'm waiting for my gas block and bushings to show up. I'll likely do a shooting report in a new thread after the fix. Good shooting!

 

[Mike 56]

I have a ASI bushing in my Mini but i have Buddy's that made their own. They removed the factory bushing went to the hardware store and bought a roll pin that fits tightly in the bushing and they tap it in with a punch with Lock-Tight. The size is about right they have a few thousand rounds though their after doing this mod with no problems.

Here is a how-to link for Mini 14s lots of do-it yourself mods http://www.perfectunion.com/vb/forumdisplay.php?f=130

Mike