Hi guys,
I'm no expert, & I'm sure someone will jump in here to clarify that admitted fact but: don't go messing with your regulator unless you have a tester, chronograph, or at LEAST an accurate depth gauge as a few thousandths deeper or shallower in that plenum will throw your optimized factory settings out the window.
What you want, is obtained by a few ways. One, being, deal with the drop. What's significant to you? I have all of my rifles zeroed dead nuts on at 32 yards and at 100 yards I can have 11" or more of drop from my close to 30ftlb .22 and certain pellets. Going to cast slugs is going to be like you threw a slider on purpose. Without removing that regulator & having a custom Dan Brown huge one installed, it won't happen. If you were to spend that money, then it's make NO sense not to get a longer barrel of higher quality MEANT for slugs. I believe your Crown barrel is choked, which means, it is NOT meant to shoot slugs and having any luck at all with them surprises me.
If I were in your shoes and absolutely had to shoot slugs & pellets to 100 yards with 1" or better precision, this is what I'd do. Call TJ's Liners & get a custom longer hybrid barrel. No choke and a fairly tight bore for pellets that DO vary in size quite a bit right out of the tin. Now, I don't know .30 (basically heavy .22LR weight) optimal twist rate but I happen to know 1:16 has been an accepted standard for a long time. Take Aguila SSS 60 grainers. Neat little pills. Good luck unless you get a special 1:12 for them. Then they are something to behold. These are all very near air gun abilities, crossovers at this time in the air gun world.
So, your choke is slowing those cast slugs down. Your limited barrel length, regulator, all settings (hammer weight, preload, TP, etc) are set for what the rifle came with on it barrel wise. If you want a regulator and a rifle that spits both pellets & slugs, you'd probably save a lot of frustration by sending it to a specialist that has the equipment, time, experience to set her up to do so. It really comes back to do I accept this rifle for what it's set for? Do I spend money to modify it by myself or another? Do I buy a different rifle that's meant to spit those 65 grain Nielsen .30's into a 1" group at 100 yards or much further & not much bigger pattern?
You said significant drop. Gravity is unavoidable, you know that. I'm just wondering how accurately the rifle is dropping them at 100 yards and actual drop in inches. To me, drop is just part of air gunning. What kind of power is retained & accuracy is what's important to me. I don't care if I have to deal with 8-12' of drop as long as the pill is still stabilized & carrying enough energy to kill what varmint I'm aiming at. To me, that's the entire fun of air gunning. Gauging the drop & windage. If I wanted a laser, there's .223 WSSM, .204 Rugers & many other screamers that are literal lasers to near 300 yards. Air guns are NEVER going to appeal to that crowd, yet on the other hand, explain to them that shooting an airgun at 200-400 yards is a lot like them firing to 1000, maybe it would click that air guns are the best teachers of learning how to read wind & gauge drop by sight, or with modern tools (ranger, Kestrel) but don't fear $1.00-near $5.00 misses when you can get 150 almost match grade pellets for $18.99. Just rambling. Hope I helped somewhat.