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Centerfire Rifles - Semiautomatic or Gas Operated Centerfire rifles, carbines and other gas operated rifles.

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  #1  
Old 08-14-2012, 9:33 PM
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Default Direct Gas Impingement vs. Piston operated

Looking to get my first AR. So a couple of newbe questons.
What is the difference between direct gas impingement and piston operated ?

What is A2 uppers and A4 uppers, in general what does A2 or A4 refer to? And any suggestion for a good AR in the $800 to $1,400 range.

Thanks.
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  #2  
Old 08-14-2012, 9:42 PM
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About $300 or so, depending on the system.

Direct gas impingment taps off gas from the barrel, runs the gas through a hollow gas tube, and using the bolt group as a piston, directly impinges the bolt group causes it to drive backward which is what makes it semi-automatic.

GPUs operate in either short stroke or long stroke. Without going into the differences between the two; it taps off gas from the barrel, runs the gas through a gas tube into a waiting gas piston, the gas impinges the piston sending an operating rod that may or may not be attached to the bolt group itself, the operating rod forces the bolt group rearward which is what makes it semi-automatic.

A1/A2/A3/A4 refer to the upper style. A2 uppers utilize the older shallower feedramps and a fixed carrying handle. A4 upper utilize the newer "M4 style" feedramps that are cut deeper and also utilize a standard rail atop the receiver.

I'd strongly suggest BCM, Colt, Daniel Defense, or LMT. Top shelf manufacturers, simple, and all AR parts fit their uppers. GPUs have no standard, all manufacturers are different, you are essentially married to the company you choose, many standard AR parts are not compatible, more parts that can go bad, break, or need replacing/maintenance, and more complicated.
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Old 08-14-2012, 9:49 PM
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Go DI you could build a hell of an AR for $1400

The main difference between DI and Piston is the way the BCG is operated by gas or a short stroke piston. That and Piston AR's are currently proprietary and DI is a standard.

Just get a flat top upper you can add a A2 carrying handle if you want.
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Old 08-14-2012, 9:51 PM
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Wow! Has it been 2 weeks already since we discussed this?

Unless you are going into the big sandbox to fight badguys, DI is cheaper and works fine.
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  #5  
Old 08-14-2012, 10:02 PM
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Originally Posted by Legasat View Post
Wow! Has it been 2 weeks already since we discussed this?

Unless you are going into the big sandbox to fight badguys, DI is cheaper and works fine.
Not those comments again. Last I checked the DI has been far more widely issued, used, and more successful in the sandbox than any GPUs. The Israelis trusted it for decades. One of my co-workers is a former Ranger who did 2 tours in Afghanistan and 2 tours in Iraq and never had an issue with his AR nor witnessed one, another coworker was a Marine with 1 tour in Iraq and he reported the same, another buddy of mine I've known my entire life went to Afghanistan with the 10th Mtn and was training Afghani National Army he never witnessed AR problems in the hands of Americans or Afghanis, Afganis whom by the way according to him were exellent as losing and breaking parts, and missing the paper. GPUs have seen little success at best, being carried by some PMCs whom are not the tips of the spears like the US Soldiers and US Marines who carry DIs.

The idea that that a GPU is necessary for the ME is from fairy tale land.
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Old 08-14-2012, 10:07 PM
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Both work fine and will out live you. Its what you think works better.
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Old 08-14-2012, 10:15 PM
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If you want a piston gun, get an AK.

If you want a more accurate gun than an AK, get an AR with a free float rail or tube.

There is NO reason you need a piston AR. It's more expensive and not any better than a direct impingement gun.

Seriously - DI is more accurate. Go DI if you want an AR.
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Old 08-14-2012, 10:16 PM
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The idea that that a GPU is necessary for the ME is from fairy tale land.
Agreed
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Old 08-14-2012, 10:41 PM
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Is there any proof that DI is more accurate than piston or is it complete bs?
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Old 08-14-2012, 11:04 PM
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why does everyone say a piston is more expensive?

I paid $609 for a complete Adams arms upper.

Compare that to $439 for a BCM that doesn't include a charging handle, bolt carrier group and handguards.

Which one is cheaper?

Next people will tell you that if your stranded somewhere in a fire fight that you wont be able to get parts for your piston upper. This makes me laugh everytime.

Some things to think about with a gas piston is the weight and differance in recoil.

The recoil is very reasonable but you do feel more in the shoulder. I put compensators on my rifle so this is a very small problem.

The weight is something that can be a turn off for alot of people. It personally does not bother me and is something that after a couple range trips I don't notice anymore.

In terms of them being less accurate I don't shoot for accuracy on a bench but i have no problem hitting the targets at angeles shooting range on the rifle side. Im sure others could help you out more with this it just is not something I have had the chance to test.
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Old 08-14-2012, 11:11 PM
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I like my piston drive AR. As whlgun said, the added weight of the piston will make the difference. With piston drive you will have a lot of time shooting than cleaning. :P
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  #12  
Old 08-14-2012, 11:20 PM
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I would love to do a filthy 14 style test on my adams arms. I just dont have the money to buy all that ammo.
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Old 08-14-2012, 11:22 PM
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Here we go again..
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  #14  
Old 08-14-2012, 11:39 PM
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I prefer piston.

Piston is a cleaner, more elegant design.

DI works too. Proven over the decades.

Read the Guns & Ammo article. That about sums it up.
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Old 08-15-2012, 4:13 AM
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DI - Great all around, light, little to no proprietary parts, needs extra TLC when shooting suppressed/SBR/automatic

GPU - Heavier, proprietary parts, excels at shooting SBR/suppressed/automatic/all three together, few "great" brands(LWRC, HK, ADCO big 3), cleaner running

As for the accuracy, it was explained to me that with the piston parts in the GPU can affect barrel harmonics thus throwing long range shots off but it is mostly prevalent in low end GPU rifles or cheap conversion kits. It's a non issue with the "higher tier" brands.

Does the typical shooter need a GPU, not really. It doesn't do much better than a DI and the main pros of the GPU system is negated in Kali. That said, I own both but my piston is my go to rifle.
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Old 08-15-2012, 4:23 AM
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Quote:
Originally Posted by phrogg111 View Post
If you want a piston gun, get an AK.

If you want a more accurate gun than an AK, get an AR with a free float rail or tube.

There is NO reason you need a piston AR. It's more expensive and not any better than a direct impingement gun.

Seriously - DI is more accurate. Go DI if you want an AR.
This. You'll get MUCH more for your money with a DI carbine, than a piston model. And this from a former POF-415 owner.

Nice rifle - THOROUGHLY UN-necessary. Stick with DI.
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  #17  
Old 08-15-2012, 5:09 AM
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DI is old technology and on the way out.

Piston is the future.
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Old 08-15-2012, 6:26 AM
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Anyone that believes a piston AR15 is superior to the DI AR15 has fallen for the marketing hype.

A rifle build as a piston with the intention of being a piston gun in the first place is just fine. i.e. M14, SCAR, etc. Not knocking piston guns at all.

However, an AR15 with a piston in it is just STUPID. If you want a piston rifle, go BUY ONE MADE TO BE A PISTON TO BEGIN WITH.

It amazes me that people fall for the hype and BS. Then again, my guess is those people don't really understand how the AR15 was designed to operate to begin with.

I think someone said, "Is the DI more accurate or just BS"? Yes, the laws of physics dictate that with all things being equal, the DI will be the smoother operating, less sloppy recoil rifle.

And BTW just so you know, DI is the "NEW" technology, piston is the "OLD" technology and the reason your friendly gun shop owner pushes the piston is because they make more money off you.
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Old 08-15-2012, 6:31 AM
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Guys, Thanks, this is very informative. Sorry if this is a question that is posed often on the center fire forum, I hang out on the pistols forum so I get the idea that my questions are often seen here much like the "which is better 9MM, 40 or 45" on the pistols forum.

I was leaning toward a Sig AR, GPU, I have been hitting all of the manufacturers web sites that I could find as I leaf through Shotgun News. Worst thing out most of those sites is finding out which ones are Cal compliant, very disappointing. What I do want is probably a 16" barrel, floating hand guard and a flat top with flip down front and rear fold down sites, so I can add some simple optics. What about triggers, I see some 2 stage and then the others, what works the best ?

I have fired an M16 on 3 round burst and an AK on full auto, wow what a difference. I'm an engineer so I appreciate fine machines. The AK rattled (my best description) and the M16 hummed.

Last edited by BrassCase; 08-15-2012 at 6:35 AM.. Reason: Add a little more
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Old 08-15-2012, 6:42 AM
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Fine, high tolerance super precise machines... get yourself a Larue and be done with it. Of all ARs I've ever owned.... of all i've ever shot... Larue Tactical has impressed me the most. It's made to a different standard.
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Old 08-15-2012, 6:45 AM
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I have a GPU. The ONLY advantage to it in my opinion is that you get a lot less (virtually none) carbon build up when using cheap ammo. After 300 rounds, it's cleaner than than about 30 rounds through a DI. That is the only advantage I've seen. Since I actually enjoy cleaning my guns, cleaning is not a big issue. The only reason I got the GP is because I got a good deal on it second hand.
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Old 08-15-2012, 6:52 AM
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Quote:
Originally Posted by CK_32 View Post
Here we go again..
This subject needs to be a Sticky.
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Old 08-15-2012, 7:07 AM
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Finally a thread that will puts the whole DI vrs GP argument to rest. I am so greatfull.
Here's a novel approach;
Load weapon
Charge weapon
Fire weapon
Bullets strike where you aim
Some minor part may need replacement due to wear after 6k or 10k rounds
Lets call this a good weapon regardless of weather it farts from the front or back. (GP vrs DI).
Lets start comparing GP to GP and DI to DI. An apples to apples aproach.
Carry on.
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Old 08-15-2012, 7:07 AM
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Quote:
Originally Posted by whlgun View Post
why does everyone say a piston is more expensive?

I paid $609 for a complete Adams arms upper.

Compare that to $439 for a BCM that doesn't include a charging handle, bolt carrier group and handguards.

Which one is cheaper?

Next people will tell you that if your stranded somewhere in a fire fight that you wont be able to get parts for your piston upper. This makes me laugh everytime.

Some things to think about with a gas piston is the weight and differance in recoil.

The recoil is very reasonable but you do feel more in the shoulder. I put compensators on my rifle so this is a very small problem.

The weight is something that can be a turn off for alot of people. It personally does not bother me and is something that after a couple range trips I don't notice anymore.

In terms of them being less accurate I don't shoot for accuracy on a bench but i have no problem hitting the targets at angeles shooting range on the rifle side. Im sure others could help you out more with this it just is not something I have had the chance to test.
I would pick the BCM if my life was dependent upon long term reliability. I bought my first AR in the early 70 s and always wondered why DI? But they work good enough if you keep them lubed . IMO I like the AK piston design

Last edited by Sunday; 08-15-2012 at 7:10 AM..
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Old 08-15-2012, 7:10 AM
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Quote:
Originally Posted by rogervzv View Post
DI is old technology and on the way out.

Piston is the future.
Then why were piston guns made before the AR? If you're going to claim that one is old and the other is the future, try to make sure you at least have some sense of the timeline of development for guns like say... the ar-15 and ak-47.
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Old 08-15-2012, 7:13 AM
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Quote:
Originally Posted by HK Dave View Post
Fine, high tolerance super precise machines... get yourself a Larue and be done with it. Of all ARs I've ever owned.... of all i've ever shot... Larue Tactical has impressed me the most. It's made to a different standard.
Tolerance is the allowed variance of the blue print clearances. So a high tolerance = sloppy machine work.
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Old 08-15-2012, 7:18 AM
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Quote:
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Tolerance is the allowed variance of the blue print clearances. So a high tolerance = sloppy machine work.
Ok... low tolerance then.
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Old 08-15-2012, 7:27 AM
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Quote:
Originally Posted by Josh3239 View Post
Not those comments again. Last I checked the DI has been far more widely issued, used, and more successful in the sandbox than any GPUs. The Israelis trusted it for decades. One of my co-workers is a former Ranger who did 2 tours in Afghanistan and 2 tours in Iraq and never had an issue with his AR nor witnessed one, another coworker was a Marine with 1 tour in Iraq and he reported the same, another buddy of mine I've known my entire life went to Afghanistan with the 10th Mtn and was training Afghani National Army he never witnessed AR problems in the hands of Americans or Afghanis, Afganis whom by the way according to him were exellent as losing and breaking parts, and missing the paper. GPUs have seen little success at best, being carried by some PMCs whom are not the tips of the spears like the US Soldiers and US Marines who carry DIs.

The idea that that a GPU is necessary for the ME is from fairy tale land.
I have heard the same, yet run into those who will pound their fist repeatedly saying the M16 is still failure prone daily but have never served one day in the military...time for this urban myth to be put to bed. The bad powder of vietnam causing jams was a long long long time ago.

That said, a gas piston is easier to clean in the bolt area, but probably not as accurate due to more moments and moving parts in the assembly.
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Old 08-15-2012, 7:38 AM
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Gas Piston ARs are easier to clean the DI ones. They also require less lubrication.

If those items are high on someones priority list, then a GPU might be for them.
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Old 08-15-2012, 8:00 AM
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Quote:
Originally Posted by phrogg111 View Post
If you want a piston gun, get an AK.

If you want a more accurate gun than an AK, get an AR with a free float rail or tube.

There is NO reason you need a piston AR. It's more expensive and not any better than a direct impingement gun.

Seriously - DI is more accurate. Go DI if you want an AR.
^
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Old 08-15-2012, 8:11 AM
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Quote:
Originally Posted by Sniper3142 View Post
Gas Piston ARs are easier to clean the DI ones. They also require less lubrication.

If those items are high on someones priority list, then a GPU might be for them.
The BCG on a Gas Piston AR is easier to clean. (Although my Youngs and Larue BCGs are usually a very simple wipedown)

The Gas Piston AR adds a new cleaning factor... which is the piston itself. The DI has no piston to take apart and clean.
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Old 08-15-2012, 8:12 AM
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Sir, there are some very good posts already, esp the first reply to your question. I will add that I own both DI and GP. I purchased a GP purely to have an example of it in my collection. The AR platform was designed as a DI from the get go. Round bolt carrier reciprocating in a round reciever, no rails. If you look at rifles designed from the ground up as a GP rifle, you will notice the bolt carrier rides on rails of some sort. Ie SCAR, sig 55x series. There are maybe some that don't as I haven't examined them all obviously. Anyway, an initial problem with GP AR design was something called carrier tilt. When the piston pushed on the gas key on top of the bolt, it caused the rear of the carrier to tilt down and make undesirable contact with the receiver extension. This "issue" has been addressed different ways by different mfgs, some more successfully than others. In the end, the AR platform wasn't designed to be GP.

All that said, I do like my GP rifle (HK 556). I don't like it more than a DI, but I do like it and it works as advertised. It would certainly NOT be my first or one and only. For me, a first or one and only would be a DD, Colt, LMT, or Bravo rifle. If I was really strapped for cash, then S&W mp sport. That's just one guys opinion. Many here will disagree and tell you to roll your own. For a person brand new to the rifle, I personally disagree with doing that.

Try and learn to use your sights before going optics, and I highly recommend an Appleseed weekend to hone your marksmanship skills and become intimate with your new rifle.

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Old 08-15-2012, 9:31 AM
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I dont understand why DI fans feel so strongly that gp is useless.

I would say that piston may not be the future but it isnt going anywhere. DI fans will just have to get used to it.

If you like shooting cheap ammo a gp is better period. For me that makes it worth it.
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Old 08-15-2012, 11:02 AM
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Quote:
Originally Posted by bighead View Post
Is there any proof that DI is more accurate than piston or is it complete bs?
Not BS, a gas piston system has a ~large weight cycling above the bore it effects the resonance of the barrel and adds a force that is not coaxial to the bore.

That doesn't make it inherently inacurate but it's not as good as a free floated barrel with a small gas block and a thin gas tube with only the weight of some gasses cycling above the bore.

You'll never see a match rifle AR with a gas piston system.
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Old 08-15-2012, 11:15 AM
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Quote:
Originally Posted by wash View Post
Not BS, a gas piston system has a ~large weight cycling above the bore it effects the resonance of the barrel and adds a force that is not coaxial to the bore.

That doesn't make it inherently inacurate but it's not as good as a free floated barrel with a small gas block and a thin gas tube with only the weight of some gasses cycling above the bore.

You'll never see a match rifle AR with a gas piston system.
+1

To add to that and take it a step further, competition shooters often times reduce the reciprocating weight, even in the carries. Combine that with an adjustable gas block and you have a very soft shooting and reliable action. It also helps promote accuracy. Just look at the products JP Enterprises makes for example.

Last edited by ExtremeX; 08-15-2012 at 11:19 AM..
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Old 08-15-2012, 1:14 PM
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This info has been great. My other rifles are bolt action or pump (Remington Gamemasters (#141) in 30, 32 & 35 cal) so an AR will be a nice addition. Using the advice and explanations all of you have been so kind to provide, I'm looking for a Colt LE6940CA. It is a DI with flip down front and rear sights. I've called around and it seems to be back ordered everywhere. Everyone wants a "Black Rifle". I had thought about going the AR10 308 route but ammo for it is expensive, the AR15 will get the job (mostly targets) done I'm sure.

Last edited by BrassCase; 08-15-2012 at 1:15 PM.. Reason: Typo
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Old 08-15-2012, 1:16 PM
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The Colt 6940 is an excellent choice. You will enjoy it greatly.
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Old 08-15-2012, 2:07 PM
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You are making a fantastic choice for a first AR. Good deal. Best of luck finding one. Try calling Tracy rifle and pistol. Mike gets colts in from time to time. The nice thing about getting that colt, aside from it being a very nice rifle, is if you ever want or need to sell it, it will move quickly for little loss. Have a great time with it!
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Old 08-15-2012, 4:37 PM
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Quote:
Originally Posted by rogervzv View Post
DI is old technology and on the way out.

Piston is the future.
Not true.

The first adopted DI rifle was the MAS40 in 1940. By then, we already had the M1 Garand, the BAR and SVT38 among the gas piston rifles adopted by armies (the Garand and BAR by us while the SVT by the Russians).

To the OP, you can get a DI along with a chromed or Nickel-Boron BCG and cleaning will be a breeze: just wipe the BCG and clean the chamber and boresnake the barrel.

Last edited by BHPFan; 08-15-2012 at 4:42 PM..
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Old 08-15-2012, 5:48 PM
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Quote:
Originally Posted by rogervzv View Post
DI is old technology and on the way out.

Piston is the future.
This was the craziest thing i have read all week.


The DI system used by the French derives from the Rossignol ENT 1900 and 1901. then used in the MAS STA 1922, 1924, 1935, 38/39, MAS 40, MAS 44 A, MAS 44 B, MAS 49, MAS 49/56 and MAS 62. It is a very reliable system
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