Factory Gas Rams

Your right it doesn't make sense.Buy the gas ram in the rifle already.

As fair as I know after market gas rams are the same as what is installed by the manufacturer. But different manufacturers use different types of gas.

Depends upon the particular maker, but I would generally opt for a factory-installed gas ram if that is what you want. If you have a springer laying around that you want to have converted to gas ram, certain sites (PA comes to mind) will install them for you, provided that the piston and the gun are compatible.
Crosman (and Benjamin) use the NP and NP2 gas cyclinders, and I believe Gamos are compatible with it. This is the gas piston most commonly available for conversion projects. I'm thinking of having one installed in my old Silent Cat.
Umarex (and Remington branded guns) use the ReAxis piston, which is mounted "backwards" as compared to most other cylinders.
Hatsan (and another brand that they make, I want to say Stoeger) uses the Vortex (or Vortek, depending on who is writing the description) piston.
When in doubt, do some research on the gun you want converted. If buying a new gun, it's worth the extra cash to just get the gas ram model.

 Cant help you with the gas ram thing, most people like them. I have an RWS92 which would have been the comate 220 I believe , advertised at 700fps, shoots 7.9g at, 700fps! Good trigger after slight work, a very good barrel. I still haven't personally seem one of their pre-charged rifles but have fired several of their sprin gers, all good.

John

Stick with a spring.more power.

There a lot reasons to buy a spring gun verse a gas piston gun, Is one better over the other? NO, Is one cheaper over the other? NO, Is one more tunable over the other? YES, the spring gun will be more tunable. The question is are you a tuner? If not the gas piston gun will offer less fuss. There is a reason so many guns are going to gas pressed over spring. There gas piston should last longer keeping the fps of your gun close to the same over longer shot cycles. We're the spring will breakdown over time and cycles. Now I'm meaning years and thousands of shots. What I'm trying to say is, pick a gun that you like. There will be the next gun that grabs you eye or the better gun next time. That what fuels the love for the sport. If we were to get the last gun, best pellet the first and last time. Few would still continue to enjoy shooting. good luck Monte

"Mrshosted"There a lot reasons to buy a spring gun verse a gas piston gun, Is one better over the other? NO, Is one cheaper over the other? NO, Is one more tunable over the other? YES, the spring gun will be more tunable. The question is are you a tuner? If not the gas piston gun will offer less fuss. There is a reason so many guns are going to gas pressed over spring. There gas piston should last longer keeping the fps of your gun close to the same over longer shot cycles. We're the spring will breakdown over time and cycles. Now I'm meaning years and thousands of shots. What I'm trying to say is, pick a gun that you like. There will be the next gun that grabs you eye or the better gun next time. That what fuels the love for the sport. If we were to get the last gun, best pellet the first and last time. Few would still continue to enjoy shooting. good luck Monte

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The vast majority of high end springers continue to use steel springs and there is no sign of that changing.

On the criteria of reliability, longevity and tunability, I think that steel springs are a better choice than gas rams. 

Even the highest quality gas rams can leak after years of use, and they are not easy or cheap to service.

R

Gene if you are a hunter one thing to think about is that you can leave a gas ram cocked for extended periods of time. Many times when you jump a rabbit or don't won't to be seen by a squirrel moving its nice to have your rifle cocked and at the ready. You can not do that with a springer without weakening the spring. Straight Shooters rates the Cometa Fenix 400 very well but I did not see it being offered in a gas spring version there or at PA. Bill

No one will like my answer...

To date, I've owned four. Forget the brands, models, and calibers, as they all have the same issues.

Most come with a scope, but that scope is mounted on the action, not on the barrel. As a result, even he slightest movement in the barrel to action connection will cause accuracy issues. This says nothing about the loosening of the action to stock mounting screws!

Recoil is at the top of the list, no matter the make. Without a VERY firm and exacting hold, they shoot like a scattergun! 

Finding a pellet make/weight/design which will shoot accurately in any specific make/model, is a hit and miss proposition. Universally, manufactures tout their velocity. It really doesn't make much difference how fast it is, or how much muzzle energy it has. If you can't hit what you're aimed at, what difference does it make? 

"AirgunBill"Gene if you are a hunter one thing to think about is that you can leave a gas ram cocked for extended periods of time. Many times when you jump a rabbit or don't won't to be seen by a squirrel moving its nice to have your rifle cocked and at the ready. You can not do that with a springer without weakening the spring. Straight Shooters rates the Cometa Fenix 400 very well but I did not see it being offered in a gas spring version there or at PA. Bill

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There have been many controlled experiments which show that leaving a spring cocked for hours has no ill effect on power. 

The timeframe has to be measured in days to result in measureable change.

R