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Author Topic: Daisy 790 max power modification  (Read 348 times)

Offline Mintie

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Daisy 790 max power modification
« on: July 18, 2018, 02:31:44 PM »
To start with I know I am modifying a somewhat rare and desirable pistol but I hope to keep the mods reversible, and I already have the more desirable S&W 78G anyway :)

This pistol I will be modifying is a Daisy 790 that I found in a pawn shop (along with a 78G, 79G and a powerline 41!) and had on sold to a friend a few years back, I asked him recently if he was using it much and found out he had also purchased a nice Diana recoiless pistol which he prefers so he hadnt used the Daisy. I organized to buy it back off him as I had been having evil thoughts of modifying the 78G and thought it would be a much better idea to leave that well alone!

I picked the pistol up last week, while my friend owned it he has purchased a new valve stem and seals off Youngs so it was holding gas perfectly and putting some Geco pellets on paper it seemed to be grouping quite well too! 13.1mm ctc at 10m standing. My chrono shows its doing 425fps on light weight 6.9gr pellets for 2.7fpe or 385fps on Jsb heavies for 3.4fpe. This is without me touching the power adjuster. The S&W 79G I have owned in the past was hitting 570fps for 5fpe so the HST adjuster on this Daisy may have been turned down a bit.

Closer inspection of the Daisy showed that the pellet loading tray has been ground out a little at some point in its life and the probe wobbles side to side a bit because of this, I dropped a 25cal pellet in the tray and hey presto it was a perfect fit! This was meant to be. I am now on the lookout for around 10" of 25 cal barrel to machine up to fit this pistol, Im guessing the stock valve wont flow enough and the stock hammer might not be able to hit the valve hard enough either but I will start with the barrel and see what sort of performance I can get just be putting the larger cal on it and work on the rest from there.

My past experience with sody has shown that the more time the co2 has to expand to its full potential the more power it will produce, for example a longer barrel will allow the co2 to expand more before the pellet leaves the barrel so pushes it faster, or a heavier pellet takes more time to leave the barrel so allows more time for the co2 to expand. This is why I am hoping that a 10" barrel (stock barrel on these is 9" but I can hide a little bit inside a suppressor etc) in 25 cal will get the most power possible out of this beautiful sody pistol. Aiming for 12fpe but I have been told that's a little ambitious!
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Offline Akzle

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Re: Daisy 790 max power modification
« Reply #1 on: July 19, 2018, 05:30:59 PM »
I will be watching with interest - I have one sitting in one of my safes somewhere.

A few things help CO2. An expansion chamber (where the gas can warm before being used), long barrels, and a port'n'polish - smooth all the airpaths. IIRC, these take a couple of 90 degree bends and flow (nominally) through the bolt probe, which isn't ideal. - There's plenty of slop for a larger probe (= larger bore) and smaller o-ring. There was a mod where you replace the "needle" forward of the o-ring with a loop of wire to further increase flow.

Offline Mintie

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Re: Daisy 790 max power modification
« Reply #2 on: August 02, 2018, 05:49:44 PM »
Well that's the hard part done! Got talking to Ron while in his shop yesterday and he found me a 25cal barrel in his parts bin. It came from a turk Webley Patriot and should do the job nicely. He was a good bugger and sold it to me for $30!
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Offline Mintie

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Re: Daisy 790 max power modification
« Reply #3 on: August 14, 2018, 04:16:27 PM »
Had a good look at this project and decided to derail it. The wall thickness of the barrel would be far too small and profiling it down that thin would more likely than not do some serious damage to the bore. Opening up the hole in the top of the pistol wasn't viable either.

Dont worry - I still plan to use this barrel on a sody to see what power a 25 cal CO2 can achieve. I will probably create a Crosman 2550.
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