I have a transferable sear that was approved a few day ago and I’m considering my hosting options as I go forward. I have an SP89 which is still a pistol and an SP5K which is SBR’d. I’m thinking I may sell one to get a full size MP5 but I don’t know which. My preference would be to keep the SP5K since it’s already engraved and stamped but I’m not sure how well the sear will work in this. I keep reading that new full size SP5’s aren’t sear ready but am having trouble finding out whether my older SP5K will have the same troubles.

Neither the SP5, or SP5K are sear ready, and making them so depends on how deep you are willing to wade into the bog of legal interpretation.  Bring your Ouija board.

Both the SP5 and SP5K have sear blocks, and most are afraid to remove them.  There is a workaround, but it's a bit kludgey.  Some might argue that they'll need US parts added to avoid 922(r) issues.  Mike Otte (Micheals Machines) will do it though, if you don't feel like doing it yourself.  The shelfs are generally considered to be in the wrong place on the SP5s, so most have an old style shelf added and do a re-mark since welding on the receiver mandates a refinish anyway.

Some people think the SP89s are more desirable as they don't have a sear block, don't need 922(r) parts and are generally a stronger flex than other options.  They are easier to drop a sear in, just buy an un-neutered bolt carrier, drop it with your pack and rock and roll.  Hipsters think you need to put a PDW barrel in, but you really don't.

It sounds like you have a more than a few nickles to rub together.  Keep what you've got, and buy more hosts.

The SP5K is easy as pie to use with a Sear, it has a nub on the top preventing a factory FA carrier but it doesn't have anything preventing modding the existing carrier to run FA.  Send your carrier to Curtis Higgins and for $85 shipped he will weld it up and send it back as a FA carrier.  http://sandharms.com/. That and a clipped and pinned trigger housing for your sear pack puts you in business.

The SP5 is a problem since it has a carrier block in the bac preventing a FA carrier.

Which SP5K? The earlier pre-PDW models have a different auto carrier block than the SP5K-PDW and SP5. It’s a cylindrical nub that projects into the receiver from the rear sight base. These early SP5K are easy to use as a sear host. These require a FA carrier milled to fit the blocking nub.

Just to clarify, the SP5 and SP5K are completely different blocks.  The K has a nub on top, which prohibits an unmodified full auto carrier, although if you shave a small amount of material off the top of a factory FA carrier it's possible to use.  The other option is to weld a trip on the standard carrier.  

The SP5 has a piece of material in the bottom/back of the receiver that blocks a FA trip on a carrier, but no nub. Without removing this block a FA carrier will not work.  There are two workarounds, a screw in trip that you install when the carrier is in the gun or the aforementioned legally dubious option.

As mentioned above the SP5 has a "nub" that prevents the installation of a factory full auto bolt carrier. The SP5K has a special semi carrier with a flat top to bypass this block and no sear trip on the SP5K carrier.

The SP5K full auto carrier "nub" is easily bypassed by either adding a sear trip back to the SP5K factory semi carrier, buying a "full auto" RCM SP5K carrier, or milling the top of a factory full auto carrier all of which will bypass the block.

The SP5 has the block which is more like a "wing" in the back by the rear takedown pin that aligns with where the trip ramp on a full auto carrier would be so a full auto carrier won't slide by as the block is in the way of the trip.

In order to bypass the rear block on an SP5 you need  a bolt on sear trip that installs once the carrier is in the gun (TSC made these).  

Of the handful of SP5s I have seen in person (including mine) the wing style rear block is over to the side enough and/or welded in at an angle enough that you can mill the sear trip ramp on a full auto carrier narrower to bypass the block  but leave enough material to still actuate the sear trip.  This is what I did with my SP5 to run a sear on it.  Depending upon the specific gun and how much material from the carrier you have to remove so the carrier will slide past the wing block, you may need to adjust the sear trip arm so that its over to the left a touch more to make better engagement with the reduced size trip ramp on the milled bolt carrier.

I would caution either removing the upper nub block on the SP5K or the rear wing block on the SP5 unless you install a full sized 9X style shelf first.  

The legal issue in my mind is that the semi "shelves on the SP5K and SP5 are much smaller than a normal 9X semi shelf.  As a result the semi-shelves on the SP5 and SP5K don't really protrude into the grip frame in any meaningful way as the SP5/K shelf depth is basically within a couple hundredths the same thickness as the sheet metal of the grip frame.  On my SP5Ks and SP5 the shelf is ~ 0.08" deep and the grip frame sheet metal is ~-0.5".    

That results in a ~0.03" protrusion of the semi shelf into the grip frame, which isn't really enough to prevent a factory full auto pack from being installed onto the SP5 or SP5K and the rear end cap or stock installed so the other carrier blocks are critical from a legality standpoint to keep the receivers semi auto.

An traditional HK9X semi-shelf is roughly double the depth of an SP5/K shelf at ~0.16" so a full auto pack is not even close to installing to the receiver.   To me (and this is currently of debate)  installing a full sized 9X shelf on an SP5 or SP5K puts the guns are on the same legal and mechanical footing (imho) as any other 9X variant and no longer require the other blocks to keep them semi auto so you could then modify or remove them.  Granted you probably kill your resale value in the process.

Both the SP5K and SP5K-PDW have the same nub and flattop carrier to clear the nub.  It’s the SP5 that has a different setup.  Both Ks are the same.

Depending on who you believe, the bolt carrier mods would be an illegal conversion device and just as bad as the block removal.  I'm not a lawyer or even a RKI but I think the MM conversion would be ok, but I'm not about to risk my freedom or gun until someone gets a letter authorizing it one way or another.  In the meantime I've got other hosts to shoot with my sear and I'll just leave the SP5 stock until it gets sorted out.

Nobody seriously believes a BCG mod is an illegal conversion device.

It is being "designed and intended solely and exclusively, for use in converting a weapon into a machinegun?"

Do you believe that FA carriers (factory, welded, or otherwise modded for a trip) are an illegal conversion device?

FA carriers are common in many different firearms.  On an MP5 they don't work without a trip lever in a FA pack.  I don't see many people being arrested for FA m16 carriers nor anybody suggesting it's a potential issue.  Otherwise someone should tell Colt all the 6920's they've sold with FA carriers are machine guns.

It's a far cry from hacking off a piece of the receiver like Mike Otte is doing.

As far as parts are concerned, factory parts such as F/A carriers are not regulated, since they are not conversion parts.

The law does however, regulated conversion parts.

The modded carriers are "designed and intended solely and exclusively, for use in converting a weapon into a machinegun" and thus are conversion parts.

They are also a "combination of parts designed and intended, for use in converting a weapon into a machinegun"

So the legal issue is, can you now make an unregistered conversion part, to be used with a pre-86 registered conversion part? Or must all of the conversion parts have been made pre-86?

Anyone have Case Law on that, please post.

Since only one person is modding the carrier, I think the answer to your question "Nobody seriously believes a BCG mod is an illegal conversion device." is yes, just about every other HK smith thinks it might be, and thus is not doing without further guidance from ATF.

I don't follow your logic here.

There are an awful lot of smith's welding carriers, and have been doing it for decades.  Never been an issue.  There's only one that I know of doing a screw in trip for the SP5, but that's more because it's not exactly a profitable endeavor.  Wouldn't surprise me if RCM came out with a version, since at scale it would be more profitable.  I've talked with several smiths about this mod, some were very critical of it's execution but none said they though it might be illegal.

I would also point to the RCM FA SP5K carriers meant to bypass the nub.  Tons of those out there, and many smiths will grind down the top of a FA carrier.

I think that is because you are presuming a standard F/A part and a F/A conversion part are the same. They are not the same and are regulated differently.

A SP5K Carrier modded to slip past the block and trip a sear is designed solely and exclusively to convert a firearm to F/A.

Anyway, I am not taking a side on legality or not, dont really care, but that is the legal issue mentioned above.

No, it's because there are many smith's converting carriers to FA using various means.  So I don't know how you divine a consensus viewpoint against carrier mods out of that.

As stated, there is a legal difference between a FA carrier and a SA carrier designed solely and exclusively to convert a weapon into machine gun.

You do not seem to grasp that, despite my several references to the phrase solely and exclusively.

I hear that a couple of builders have sent off letters to get the nod one way or another on the various mods (carrier / block).  I'm not about to be a test case for anything so I'll just wait for them to sort it out with the ATF.

This is what I'm disagreeing with.  There are many smith's modding carriers.  Even if it were just one, there are many reasons why it would just be one other than an ATF opinion.

I was only aware of Otte, but quantity does not have anything to do with legality. I am sure some end user will "write a letter".

I am not aware of any case law but there is a long and storied precedent that you can make unregistered machinegun conversion parts to be used with a pre-86 registered conversion part to make a functional machinegun.

Many of the pre-86 standalone registered conversion parts require other unique parts that are absolutely designed and intended solely and exclusively, for use in converting a weapon into a machinegun.

Since we are in the HK forum, transferable pre-86 HK conversion sears are a case in point.  You cannot use a HK conversion sear in a formerly semi auto trigger group unless you modify a semi auto HK trigger frame by removing the semi-auto trigger stop pin.  This is what is commonly known as a "sear prepped" HK trigger pack.   In this case the now semi-auto trigger stop spin pin less trigger frame will allow the trigger to be actuated into the full auto position and allow the disconnector to clear the hammer.  To install and use a registered HK sear you have to remove this pin from the trigger frame and that modified semi-auto trigger frame by itself will  also allow full auto fire without a sear installed by allowing the hammer to follow the carrier into battery and discharge multiple rounds via slam fire operation.

Other registered conversion part guns are similar:

10/22 "Norrell" sears require modification to the trigger (button or lever) to lock the disconnector out of the way and the lower trigger frame has to have a sear hole drilled in it.  Norrell 10/22s also require a custom full auto bolt  that has no other purpose than to make a 10/22 into a machinegun.

FNC sear guns require you to mill and drill the lower to install the registered sear and modifications to the semi fire control group to allow the disconnector to be bypassed.

Many of the Uzi registered bolts require the barrel restrictor ring to be removed from the receiver and the trigger block in the lower to be removed.

AUG sears require the semi hammerpack to be milled out, a sear pin hole drilled, a bushing installed to cam the disconnector out of the way, and many had custom full auto style sear trips added to the bolt carrier (i.e. cocking pieces) in order to trip the conversion sear.

All of these registered conversion device guns are chock full of either unregistered modified semi-auto parts and/or custom made conversion parts whose only purpose is to convert the semi-auto into a machinegun.

If all of these non-factory full auto conversion parts have been allowed to be made and paired with their registered conversion part partner for 30+ years, I don't see how a modified HK carrier (which is a a lot more innocuous compared to some of these mods/parts) is any different and would be construed as an illegal conversion part.

RCM has been selling "modified" MP5K style full auto bolt carriers that bypass the block preventing the installation of a factory full auto carrier in SP5Ks for years. The SP5 screw on full auto trip carriers from TSC are IMHO in no way legally different than what S&H, MM, TPM, RDTS, and a myriad of other builders have been doing to SP5K carriers for years by adding (welding/soldering) a sear trip to semi-auto SP5K carriers (that also clear the block) since shortly after that guns introduction.

That said, I don't think it would be wise for somebody without a legal use for a modified HK SP5/K carrier to have one in their possession or installed in their gun if they don't have a legal registered sear or box  to pair it with.

Just my 2 cents anyway.

Otte's not modding carriers.  He's removing the carrier block in the receiver, and very deliberately NOT asking the ATF.  What he's doing is super sketchy imo.
Zenith was doing a similar thing until the ATF said no go.  
I wouldn't go near one of Otte's modified SP5's.

To be fair, he is also adding a full 9X style wrap around shelf to the guns prior to milling the rear full auto carrier block down.   I assume you probably know this but your post makes it sound like he is just cutting the block out.

That said he does leave the front push pin holes in the receiver sheet metal and retains the solid full size 7mm pin that the SP5 paddle mag release actuates on underneath the new 9X style shelf.

If I was to entertain removing the full auto carrier block in an SP5 or SP5K, I would remove the old shelf,  weld up the push pin holes in the receiver sheet metal, add the full size shelf, and then redrill the smaller ~2MM pin only going through one side and add the paddle mag back via a bushing like what is done on a standard 9X paddle mag conversion.

At that point an SP5/SP5K is identical mechanically to any other paddle mag conversion HK 9X semi auto.

I am aware of the history, but as we all know that mean little when .gov does not want them to.

Right here is a tutorial on how to make an unregistered machine gun conversion part: https://www.ar15.com/forums/Armory/HOW-TO-Make-an-RDIAS-Replacement-Trip/23-253170/

By definition, this is a machine gun all by itself. Just like a baffle is a silencer.

(b)Machinegun
The term “machinegun” means any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person.


I can no longer count the number of things that were once allowed, but are now clearly illegal despite no laws being passed to change that. And I imagine a whole lot more are about to be in the next couple of years.

This is very true, it is always a very shifting landscape, especially on the conversion devices that require so many other mods to work.

Very few of them will just "drop into" a semi auto gun with no other mods to the receiver or other parts and run.

Some are very clearly MG conversion devices unto themselves like lightning links and others like the full auto M16 "bolt" on the other board that are questionable at best.  Most fall into a gray area requiring modifications to other parts to function.

Then there are the extra "enhancement" machinegun conversion parts that work in conjunction with their registered component counterparts but are not required for their conversion device to function but are also clearly conversion parts.  Items like "HK Retimed" hammers that won't function with a factory full auto pack, modified HK burst trigger frames, modified 3rd burst groups for RLLs, lightning link protectors, or modified AUG stock latches so a conversion sear can be used in a AUG full auto stock.

As you pointed out by the letter of the law DIAS trips are also "machinegun conversion parts" yet are clearly considered wear/consumable parts and legally replaced.

Folks doing it without arrest is not the same thing as "legal". I think it is more a case of flying under radar than anything else.

It's convenient for you to ignore prior ATF letters that specifically said this was OK, even if they were not specifically addressed to Mike Otte.

So far as I have seen, Zenith has never published the full text of their letter, but the parts that I have seen are in agreement with prior letters that say (paraphrasing) 'There are several ways to make a semi auto HK clone, and you can switch from one to another so long as at no point in the conversion do you make an illegal machine gun'.

From what I have seen of the Zenith letter, they only asked 'Can we cut the sear block out of the receiver, but leave the pin hole and existing small shelf?'...and the answer they received was 'No, not unless you delete the front pin hole and put a big shelf on it'.  Zenith turned around and said 'ATF said we can't do it'.... which actually isn't what they said at all.

I think there is one other letter from 2012 to an Individual on Arf.  As you know, those only apply to the letter holder.  And often you can write the same letter 3 times and get 3 different answers.

I don't believe the Zenith letter says what you are implying.  The Zenith letter is not a perfect match to this situation, since the pushpin hole is functional on those.  It's merely visible on the SP5.

https://zenithfirearms.com/blog/removal-of-zenith-full-auto-bolt-carrier-blocks/
I don't see anything about adding a larger shelf to an exisiting receiver.  Modifying receivers is usually treated very differently from original manufacturing by the ATF.  
I am very skeptical of a secondhand receiver modification like that.  Mike Otte has been criticized and asked repeatedly about the legality yet refuses to get anything from the ATF, merely insisting that it's ok (in his usual horses ass manner.)  Several smiths have told me they won't modify SP5's like that or work on them for fear of legality.  After all, the list of Machine Gun smiths that ended up retiring over felonies is pretty long...

You might be willing to take Mike Otte's word for it, I'm not.  It may be that the ATF would be perfectly fine with it.  But for me, I'm not one to hope for a benevolent and permissible ruling from the ATF.  

So ya, I think it's a pretty grey area.  And when you are talking about a grey area with machine guns and the ATF I'd say that's sketchy.

This is the ATF letter on removing the Zenith block (which is very similar to the SP5 block) after adding a full size shelf.

Pretty much all opinion letters from the ATF, even if addressed to you personally are close to worthless nowadays given how often they change their mind on stuff.  

Granted its pretty much a non-issue at this point as the BATFE doesn't respond to written only "letters" anymore so there is no point to even bother to write them and ask.  

Theoretically they only will respond to a tech branch request if you send them an actual firearm for eval and even then they will probably never respond in any meaningful time frame even if you are an FFL.  As an unlicensed individual you will probably be put on permanent ignore.

However, the logic as laid out in this 2012 ask and the ATF response is sound in my opinion.   I don't know by what insane logic how an SP5 with a full size 9X style shelf and no full auto carrier block "is a machinegun" but a mechanically identical HK94, PTR, PCS, etc. gun is not.

Granted this isn't a letter on an SP5 and everybody's tolerance for risk is different.   I would agree the resale on any blockless gun given the contentious nature of this mod will be significantly undermined.


Attached File

That was specifically addressed in the other letter.  ATF has said specifically, it's OK, but replace the shelf before you remove the block so you don't accidently make a machine gun.  EDIT: Yes, in that letter ^^^^


No, I read the ATFs position in those letters.

Well that's dumb as hell, reading a moving target and one that is only applicable to the addressee.  

Just look at the history of the "arm brace."  We are on what, the 3rd or 4th iteration of the ATF's position?  At least one of those position changes cited manufacturers relying on tech branch opinions meant for others instead of submitting for their own.

Or the Bump stock, which was kosher until the ATF's position changed to it being a machine gun.

You're right about the "moving target" aspect, but these letters cannot only apply to the addressee. This is because of Due Process and Equal Protection under the law.

Ironically, such a letter does not even protect the addressee if it contains bad advise or an incorrect opinion. It is ultimately up to the judicial branch to interpret the law, and subsequently they must apply the interpretation equally to everyone.

I don’t think I would modify one of the new guns, if for some reason you were to get caught your sear might go to evidence room with modified SP5K. That might hurt. Use the SP89 instead.

What I think is dumb is reading a letter that addresses exactly what you are talking about, and saying it's not good enough.  Two letters, if you count the Zenith letter.

I've determined that gun laws are too unpredictable for you, and you should find a new hobby.

Well thanks for your determination.  In any case, I'm not the only one deeply skeptical of Mike Otte's mod.  

It's one opinion letter to one guy.  Zenith is just a straight no, they didn't ask about building a larger shelf.  The letter is from 2012.  How many ATF flip flops have we seen since then?  Bump stocks, braces, wipes, etc

If you're comfortable taking Otte's word and some letter you found on Arfcom go ahead.  It may be perfectly ok, in which case Otte's missing a good amount of business (including mine) by not formally clearing it with the ATF.  I have to wonder why not?

Again, seems sketchy to me and others.  

The SP5K doesn't require receiver modification, the SP5 does.

Never mind. MUCH better informed information in the other posts than I had to offer

It still doesn't have anything to do with Mike Otte.

I'm half tempted to sell my SP5 and buy a PCS clone to run with my sear.  I hate to sell a real HK but there is too much drama associated with making them sear ready.  Then on the other hand, I could just buy the PCS clone and keep the SP5, which I would never shoot or buy the $600 bolt mod, but I still have questions about it.  

1st world problems, I know.....