Using pellets when tunning for slugs?

No.

I used 21gr pellets when I was tuning for 21gr slugs. Worked just fine.

I can only speak for myself and my impact mk2 25cal, and I used jsb 34gr pellets to get both speed and consistent FPS . Then I used the valve adjuster to tune the nsa 36.2 slugs going by the groups to settle on the valve placement. Hope that helps 

I would say no because different pellets react different even a slug with the same weight doesn't have the same shape or diameter think slugs are more consistent that pellets they sure cost more.

I have done it and is fine (on FX Liners, important!), of course the best way is to use the same projectile for harmonics but IME if I tune for monsters 25.4gn I know that 23gn .217 slugs will be about the same speed usually. Specially if you are on the budget. For example FX Hybrid Slugs cost more than other slugs and I prefer to shoot pellet for tuning and then I check with those slugs...but I will not waste donno 20 o 30 of those slugs just for the crony.

Because slugs have much more bearing surface, and therefore friction, especially as weights go up, they fly slower for the same settings. While you can tune for maximum power, you won't get an idea about actual performance until you push the actual projectile. The other thing I find is slugs like faster speeds (I suspect because they are more in need of the spin speeds that come with higher velocities) and can have the opposite reaction relative to speed than pellets. For example a slug often becomes more accurate going from 800 fps to 1000 fps, while a pellet will often become less accurate. Because of all of this, I find many airguns prefer a considerably lighter slug than pellet. The pellets will often outperform the slugs, for power at close ranges (15-30 yds), but the slugs win as distances increase and they lose less velocity. They also buck the wind better which shows up again, at further distances.

When I tuned my Impact .25 700 slug barrel for the NSA 33.5 HP slugs it shot the MK2 33.95 JSB pellets very well also. I don't remember what the fps on the tune was though but it shot the slugs at about half an inch and the pellets at near an inch @ 100 yards without re-tuning. I was quite impressed by this.

I used pellets to sort me in for max performance FPS . Then i did the harmonic tune with slugs. This is 500 mm barrel 22 cal. 21 grain. Very happy with results . 

While tuning up for the NSA slugs in my Impact I used a very close weight pellet until I got close to my goal and then switched over to the slugs to dial in my final settings. It saved me a little on the slugs while making the larger adjustments. Worked well for me.

Remember i would only use pellets to tune/find the most consistence setting on the reg and hammer not for the accturly precision and hammonic tuning,

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I do it exactly like that, coarse setting with pellets, fine tuning with solids. 

Works perfect for me. 

The speed difference between H&N 25gn 217 slugs and monster redesigned 25.4gn was just about negligible in my gun. 

Ernest Rowe modded and tuned my FX Impact pictured in the following thread. For the tuning, Mr. Rowe had me purchase and mail to him an assortment of the NSA slugs I intended to use in the rifle:

https://www.airgunnation.com/topic/fx-impact-x-25-cal-power-tuned-by-ernest-rowe/

Absolutely you can! back in the day when they were 3 times more money for sure! Especially if you have some pellets laying around that don’t shoot great,

I burned 2 tins of h&n barracuda 30 grn to ruff tune a 25 pp impact, for 33.5 nsa’s, worked great and the 34 grn mk2’s are within 5 FPS of the 33.5 slugs. Dumped a few Tins of h&n grizzlys also the key for me Is to be in the ballpark weight wise if you can? I would not recommend using 25 grn kings for a 33 grn slug tune as they are at least 100fps apart. Yes, fine tune with the real thing, shoot both first so you have a good idea of the difference, just be smart about it, you might learn something along the way? Most full tunes take me 300-1000 shots anyway, good luck!

Yep I bought around 10,000 JSB Monster Redesigned pellets and use them for most of my 22 cal power tuning testing. For .30 cal, I use whatever I'm tuning for.

Where problems arise are times where I am making new valves. The valve pin stroke will sometimes be too short to push some of the slugs where I want, so at that point I use the real deal.