? Slugs — Testing How Much They Expand and How Far They Penetrate

Slugs — Testing How Much They Expand and How Far They Penetrate

Slugs for airguns won’t ever replace pellets.

But they aren’t just a fashion, either.
Slugs easily cut wind drift by more than half.
And at longer ranges they hit quarry with twice the energy.

Those are reasons enough that I want slugs (not the slimy ones, though if they are cured with lemon and served with yam and hot chili peppers, they are quite good!).

The Benefits of Hollow Points

But almost all commercially available slugs come as hollow points – and for a reason. The hollow point is supposed to expand upon impact and make a larger permanent wound cavity. This is good if your quarry’s kill zone is rather soft and fleshy, as a bigger wound produces a higher rate of DRT’s (over the F&D’s and R&D’s*). (However, if your quarry’s kill zone is a big head protected by tough bones, penetration is probably more important than expansion.)
*[F&D = fly & die  |  R&D = run * die]

If the hollow point projectile (slug or pellet) increases in diameter, so does the impact area, and so does the volume of crushed tissue as the projectile bullies its way through the quarry – leaving the permanent wound cavity.

For example: A .22cal projectile (slug or pellet) that expands to .295 increases its impact area by 66%.
And at that increase, even a 1½"-short wound cavity therefore increases by 85%!  85% more tissue crushed!
[Metric: A 5.5mm projectile expands to 7.5mm. Even a 4cm-short wound cavity is thereby increased by 85% – resulting in 85% more crushed tissue.]

Also worth considering is that a blunt tip in all likelihood does more damage than a domed or pointed tip. So, all other things being equal, wadcutters, unexpanded hollow points, and wide open hollow points (in this order) will crush soft tissue more than domed or pointed projectiles (that will tend to slip and slice through tissue).

What Helps the Hollow Point to Expand

After seeing the often poor performance of hollow point pellets – I wanted to know what expansion I could expect from hollow point slugs.

Bob Sterne (HardAir Magazine and GTA) published some very insightful articles and posts about the subject (cf. links).
https://hardairmagazine.com/ham-columns/bob-sterne-discusses-hollowpoint-slug-design/
https://www.gatewaytoairguns.org/GTA/index.php?topic=174584.msg155979642#msg155979642

Just to summarize Bob’s excellent discussion:

Some of the attributes that support expansion of a slug are as follows:
• larger HP opening
• thinner walls of the HP cup
• softer lead
• deeper HP cup
• slits (predetermines tearing points) open the HP in petals
• harder impact material (animal flesh is softer than play-dough is softer than clay)
• higher impact velocity! (as the tests will show, depending on the impact velocity the HP will “work” – or not expand)

HP Projectile Tests
Data on HP slug expansion is still scarce, but in the following posts I’ll share the Projectile Expansion and Penetration Tests .22cal that I’ve found so far.

Take the test results with a grain of salt (or a 22-grainer of lead if you like). As more test data becomes available some tests results might prove to be too optimistic, or pessimistic.

Some Benefits of the Test Results for the Shooter
• You will note in the tests that depending on the impact velocity the HP will “work” – or not expand. Knowing the muzzle velocity of a slug that your gun likes, you can do some simple ballistics to figure out at what the maximum range you still can expect reasonable expansion in the quarry.
• If you have the choice of several slugs that your gun like, based on the test results you could choose one that has better expansion, or choose a lighter (=faster) slug.
• If you wanted expansion at farther ranges, you could increase the power of your gun based on your ballistic calculations.

I thank all the airgunners who have run these sometimes very time-consuming tests, and for putting them out there for us so we could learn from them. THANK YOU!!  

Matthias

An Overview and Some Background of the Projectile Tests in this Thread

When sifting through the projectile tests (PT) I decided to leave out play-dough, clay, and water as ballistic mediums as they exaggerate HP expansion. Reading what terminal ballistician have written, I considered ballistic gel, ClearBallistics.com gel, glycerin soap, wet newsprint, and of course… – real quarry (dead or alive)!

The testers often do not give specific details of their test protocol (which kind of ballistic gel they are using), nor do they give exact data (impact velocity) or measurements (expanded size of the slug). I had to do some ballistic calculations and often measure the proportions of the expanded slugs in the images in order to arrive at the data as presented below.

All the tests include the following:
• a test designator, PTXXX (where XXX is a counter; PT = projectile test)
• the ballistic medium
• impact velocity, and impact energy
• expansion and/or penetration (in imperial and metric)
• source (and notes)