New Griffin HBRBT HP .25, lol!

Looks like a brilliant idea to me. The RBT is the way to go to reduce drag in subsonic projectiles. It has an even bigger effect than nose shape. A hollow RBT would get the benefit of less drag while minimizing weight gain in the slug. Some trucks use the idea to save fuel.

I've tried shooting 2 types of boat tails from my rifles. I own 2 of the boat tail air rifle slug molds from MP-Molds (.22 and .25). I've also purchased some of the boat tails from griffin slugs in .22 caliber. The following has been my personal experience with boat tails......

They want to be shot FAST! You would think that the BT design would reduce drag, thus increasing stability and accuracy. That hasn't been my experience. 

Now I must admit that the .25 were shot from my LW barrel with a 1:17.7 twist. I've since purchased a .25 barrel from the Russian Federation that is un-choked and has a 1:15 twist. They may perform substantially better in that barrel. But the .22 griffin slugs were shot from a Russian Federation barrel with a 1:16 twist (optimum for .22 caliber), and the groups were just ok. 

So don't set your hopes too high. I'm not trying to be a downer. I'm just trying to be a realist. 

I hope they work for you though. SHOOT THEM FAST. At least 980-1000 FPS at the muzzle. 

The reason your NSA slugs were so hard to push into the lead at the breech had nothing to do with how deep the rifling had been softened, and EVERYTHING to do with the ammo. For some reason Nick used an alloy blend on a certain batch of his swagged slugs instead of pure soft lead. I bought .22 ammo from NSA, Varmint Knockers, and Griffin. The slugs from Varmint Knockers and Griffin are made from pure, soft lead. They were heavier and longer than the slugs from NSA. They were all the same diameter. The VK and and griffin slugs easily seated into the breech. The shorter and lighter slugs from NSA were MUCH harder to seat. I'm not sure why Nick used an alloy blend. You don't even need anything but pure lead for air rifle slugs. The advantage of using an alloy mix is 2 fold. Firstly, antimony and tin have a lower melting point than lead, so they aid in how well the slug pours into a mold (cast slugs). Typically when casting, the alloy slugs pour significantly better and smoother than pure lead. Secondly, they increase the BNH (hardness) of the slug. This is of benefit when using powder burners where significantly higher velocities and super heated gasses can quickly foul a barrel if the projectile is pure lead. But you also have the power to push the harder lead down the barrel. Since air rifles have significantly less power, and there are zero super heated gasses, pure lead is IDEAL for slugs. Every wonder why pellet manufacturers don't use alloy blends on their pellets? I'm not saying there aren't lead free pellets. But the pellets exist for environmental purposes, not to increase the hardness of the pellet. I also don't know why he would push harder alloys through his swagging machine as it will definitely wear it out faster than soft lead.

If you're interested in a proper .22 barrel for your rifle with a 1:16 twist, my buddy Alex in Russia will make it for you for 1/2 the price of a TJ barrel. The land/groove is .215/.220 with a 1:16 twist. He will make it to whatever length you want, carbon fiber wrap it, thread the muzzle for a suppressor, and crown it for $185. That includes shipping! There's NO WAY you will get your TJ's for cheaper than that. If you're interested, shoot me a PM or email, and I'll pass on his info. He also has .25 acp in 1:15 and 1:14 twist. He will make any barrel for any gun you like. Just contact him and tell him he make and model, and more than likely he's already made it in the past. The 2 barrels he made for me were for AF Condors (24")

I'm still experimenting with both of the barrels I got from him. But your response validates EXACTLY what I've been preaching about the slug manufacturers not making slugs in the proper diameter for LW, CZ, and Russian Federation small caliber barrels. You're not just losing 1/3 of the power, that pressurized air is very turbulent, and it's destabilizing the slug right out of the gate because it beats the pellet/slug to the end of the barrel.

That's why I created a thread about powder coating your slugs with multiple thin layers. If you have a chance, check it out "Powder coating 101". Powder coating gives you the ability to add very thin layers of concentric girth to compensate for the insufficient diameter needed to properly seal the grooves of the barrel. With a proper seal, you use less energy to propel the slug down range, and you will achieve better groups/accuracy.