Build advice on first 6.5G bolt gun

[LRRPF52]

Twist rate will have an important role in keeping the projectile stable as it goes through the Mach 1.2 to Mach .8 speed region, often called the transonic flight region in aerodynamics, due to the anomalies associated with shock wave formation.

Some supersonic aircraft will stagnate/slow down their acceleration as they try to push through the transonic region (the F-14 was one that did that), while others will experience smooth transition and only see their Mach meter do a strange skip due to pitot tube and Venturi effect, like with the F-16. Aerodynamicists pay a lot of attention to the effects of the shock wave formation on a design so they can optimize it for certain behavior through and above the transonic zone.

Bullets share some challenges and benefits from these studies, but rifle bullets do something that aircraft don’t, namely spin at over 200,000 revolutions per minute for gyroscopic stability, since they don’t have wings or aerodynamic planes to guide them.

With tighter twist rates, you can help control stability nicely through the transonic region where the shockwave behavior starts to change.

The mach cone (supersonic shock wave) starts to alter its shape and location as the bullet decelerates. If the twist is looser, it’s easier for the projectile to suffer from gyroscopic wobble, resulting in erratic and unpredictable point of impact.

1/8 twist or tighter, depending on barrel length, works well with 6.5 Grendel in my experience at 1000yds.

1/7.5” twist works well for the shorter barrels.

Longer bullets with long boat tails suffer more through the transonic region if they aren’t spun really fast, while they do better for maintaining speed down to Mach 1.2 (from the muzzle to Mach 1.2).

My experience shooting Grendel at 1000yds and farther has usually been anywhere from 4400-6600ft above sea level, so the air is a lot thinner and doesn’t resist bullet flight as much as the soup down near sea level.

Even with my lightweight Lilja 17.6” Grendel shooting 123gr A-MAX at 6600ft, 1000yd shots on 3/4 IPSC steel is almost boring if the wind is steady below 10mph full value.

This is a combination of me and someone with a 24” 6.5CM bolt gun shooting this piece of steel at 1000yds at North Springs:


For a Grendel bolt gun, I would either chamber it around the 130gr Nosler RDF, factory 123gr ELD-M or 123gr SMK, spin it tight at 1/7.5-1/8, load it long to get more of a BR case capacity, use a Bartlein pipe in a nice micro action, and set the barrel length for the anticipated altitude region I’d be shooting. Low altitude would get a longer barrel.

If you’re planning on shooting a lot of factory ammo, going with 6.5 Grendel makes a lot of sense if you want a micro action.

If you don’t have a problem forming cases, you could shoot a 6 BR variant or 6mm AR variant. What type of magazine you use will matter as well.

6mm Micro action bolt guns are a lot of fun too.

I’d do some type of aluminum chassis and fit the build to your anatomy so the rifle points and handles optimally for you.

You can get measured b a true craftsman gunsmith, or measure your own length of pull, cheek location, eye height with comfortable neck position, wrist angles, and build around all of that.

I just enjoy shooting gas guns more and hitting steel. If I were punching paper with a range that had pits, I might choose something different.