.50 BMG Probe

I’ve seen a few references and photos of an experimental .50 BMG round referred to as LRTA Probe made by AAI in the early 1980s. One photo (from Jestertoo) is below with the round in question on the left.


I have been unable to find any description or details on these, other than the headstamps (LC 81, etc.) and the phrase “LRTA Probe” (nothing specific found on the forum or in HWS vol 3). I understand that LRTA is Limited Range Training Ammunition, however, I question whether this is the correct label for these rounds. The reason is that a similar probe was used on the Trident ballistic missile in the 1970s and referred to as an aerodynamic spike or “aerospike.” The aerospike was used to reduce drag and increase range of the missile, so it would appear to work opposite of “limited range.”

Trident Aerospike:
Source: https://en.wikipedia.org/wiki/Drag-reducing_aerospike

A NASA paper describes development of the spike (emphasis added):

The Aerodynamic Spike (Aerospike) is a deployable drag reducing mechanism stowed within the nose fairing of the Trident I submarine launched ballistic missile. This mechanism maximizes missile performance within the limited envelope available by transforming the aerodynamic characteristics of a blunt, space efficient nose fairing into a more streamlined shape.
Source: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19790014372.pdf

This description seems to fit the .50 BMG probe in appearance. Does anyone have any thoughts or information on the development of the AAI round? Could this actually be an experiment to increase range with a blunt ogive?

Below is a depiction of how the aerospike works:
Source: https://fas.org/nuke/guide/usa/slbm/c-4.htm

Larry this probe is also used on plenty of HEAT fin and drag stabilized projectiles and also on some smooth bore experimental for smaller calibers.

For HEAT rounds, isn’t the “probe” merely a stand-off detonation initiator for the shaped charge?

If you look at the projo of the AAI probe nose, you’ll understand. It’s very short, extends just barely past the case mouth into the case neck, compared to other standard .50 BMG projectiles, so the CG and CP of the bullet are not in normal association, causing quick destabilization. The ID / function of the round was confirmed by an AAI employee. The project was beyond the 1945-1977 scope of Vol III.

Think of it this way. The aerospike and the AAI probe nose both have probes, like a Ford F150 pickup and a Ferrari both have 4 rubber tires. Doesn’t mean the Ford and Ferrari perform the same roles or function the same. The probe is only a single feature in common with both.

The stand-off is also realized with conventional projectile designs as it was/is done for many decades. As HEAT projectiles are badly affected by rotation (as we know it from normal rifled barrels) such projectiles are best stabilized by fins or aerodynamically (the latter very seldom as fins work better).
Speaking of fins we need to know that caliber sized fins are not the optimum as they are not entering the airstream and sowith are providing stabilization only after the projectile already has left it’s optimum trajectory (proj axis not parallel to trajectory), again this is not desirable. This is why rockets usually have over caliber fins (folding or rigid).
To achieve a caliber-sized fin section engaging the air stream (the example I have shown above) the airstream is brought (diverted) to the fins. This is what the probe here is doing, it guides the air stream around the projectile (bypassing the large front flat which normally would cause high drag) and narrows in around the caliber sized fins and thus stabilizes the projectile on the whole trajectory and not only after the projectile has left the optimum trajectory - causing less precision.

Keith, thanks for the excellent explanation! Would you happen to know if there are any reports discussing the development of this round?

Alex, thanks too for your outstanding description of the HEAT airstream! I also thought that the stand-off was only there for the shaped charge to develop, but if that was the only purpose then it probably would have a more streamlined windscreen (as some HEAT rounds do). There is always much to learn here!

to my knowledge, none have come to light

DITTO !! Thanks for the additional insight.

" I question whether this is the correct label for these rounds"©

Yes this is correct.

Spitz has nothing to do with aerodynamics.
He has not such a form as the whole bullet.
This is necessary for the correct positioning of the cartridge in the belt mechanism.

Stiven, welcome here!

Could you explain what it is about the correct positioning in the link?

In Browning machine guns, the cartridges in the feeder are positioned resting against the feeder shifts with the bottom of the case and the nose of the bullet.
If the cartridge is moved forward or backward, the extractor may “miss” past the groove in the case and this will cause a feed failure.

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Ok, this I kinda knew, but I got it this way that you said the “probe” on the projectile is to aid positioning?

Stiven, welcome to the forum and thanks for the explanation. The function of the “probe” makes perfect sense now.