Anyone install a Huma in a Huntsman Regal XL .177??

From everything I have read, the problem will be power, or lack thereof. Based on this, if not loosing power is a priority, I would look at the new models that come regulated already. Most people don’t really see the benefit of it either. Hope this helps, I don’t have experience with doing this though. 

I can only offer a couple of related comments.

I’m not familiar with the Huma for these, but I purchased an Altaros for one of mine. Again, cant speak to the Huma, but the Altaros will only (according to their website), fit in one of the two tube styles these guns came with. If that seems important for your process, I’d go to the Altaros website. You’ll have to look around but it will show you a pic / video of what style it will/wont work in. Basically, to be compatible, the tube need to be removeable at the rear.

If the Huma is different I dont know... However I still have that untouched Altaros in my drawer.... Sold the rifle and the buyer didn’t want it.
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I installed a Huma in my 40 fpe Regal a couple months ago. Shot count was simply awful before the installation. Twenty shots with about a 65-70 fps spread. The 40 fpe model doesn't have the sling shot hammer unfortunately, so I can't offer any advice on getting the hammer spring adjusted to the regulator on yours.
Installation is really pretty easy and straight forward, setting the hammer spring was another story on mine, lots of trial and error. I made a shorter bushing that the hammer rides on trying to avoid cutting the hammer spring, still wound up cutting it down some. I'm getting 35 shots now with a fill of 3300 psi a spread of about 15 fps shoot 26-27 fpe. Not great, but so much better than it was.

Yeah, it's kind of a balancing act depending on what kind of power you're looking for. For instance, if you shot string now runs 850 to 900 fps over say 40 shots (I have no idea what you gun may be doing by the way, example only) you'd probably want to set it up in the 850 to 875 fps range. You lose some power, but gain a nice flat consistent shot string that, in this example, might push your shot count up to 50 shots or more. The lower the power you're willing to accept the more shot's you'll gain. Let's say you're going for 850 fps with 10.3 JSB's. You'll need to get an accurate reading of how much air pressure is remaining in your air tube when you reach this velocity and set your regulator at that pressure or very slightly above. Anything higher and you just wasting air and losing shot count. Then you need to reduce you hammer spring tension to match the now lower pressure required to open the valve. If you've been filling your gun to 3300 psi but you regulator is set at 1500 psi, that's a lot less hammer strike needed to open your valve. There's a lot of info on how to set one up by guys that can explain it much better than I can.
I will say this, if you're looking to stay at the upper end of your shot string a regulator is not going to be very practical for you, lower to mid power on your sting and you'll be a happy camper once you get it set correctly.
Hope this helps.

I tried the Altaros in my 22fac version. The power went way down but without any additional plenum space that's what I was kind of expecting. The good guys at Altaros at the time didn't incorporate a plenum spacer. It was worth a try though and Altaros being new to the Regal regulating wanted me to try it out and send it back if the results were not there. Maybe they have something different by now. Huma may have also came up with a design. I know that "Motorhead" over on GTA is regulating some Regals so you can always get in contact with him.
Jimmy