If the info MDog and new guy posted doesn't walk you through it, the link I posted earlier should.

Several members built AR pistols and walked everyone through the steps.

Most were semi-autos but same steps for lower.

I believe I understand what I have to do, sorry if Neglected to say thank you earlier.


Next step is to decide what caliber would be best in such a platform...

How many different calibers do they make pump action AR uppers in?  I ask cause that's what you stated in your original post.  Unless you changing plans.  What's the overall goal?

I posted the steps IN ORDER.  Maybe the problem is, you need every intermediate step spelled out for you, too?  My mistake, since you did tell me you have gone through the registration process before.  Either you're lying about that, have the memory of a goldfish, or need your mommy to hold your hand again like the last times.

1.  Go to the gunshop, find a place to park, and go inside. 
2.  Pick out the AR receiver you want to build into a pistol.
3.  Make sure the gunshop will be willing to report it as a Handgun to ATF on form 4473
4.  Pay for the receiver and keep your receipt with the serial number
5.  Go to HPD between the hours of 7:45 AM and 3:00 PM, Monday thru Friday, excluding City & County Holidays
6.  Park and lock your vehicle
7.  Make sure you park in a legal space
8.  Make sure you put enough money in the meter for the visit -- usually an hour is plenty, but you can leave and feed the meter if there's a line
9.  Make sure you have the receiver receipt, your valid Hawaii Driver's license or other valid photo ID, your safety training documentation and know who your healthcare provider is
10.  Go to the Firearms section and stand in line.  If there is no line, wait at the doorway to the firearms section to be called.
11.  When called to the window, step forward.
12.  Tell the officer behind the glass you would like to apply for a permit to acquire for a pistol.  Show him your ID, your safety training documentation and the receipt. 
13.  Make sure the officer understands this is an AR receiver you intend to configure as a handgun.
14.  Take the paperwork the officer gives you, and find a place to write.  You'll need a black ink pen.  The pen can be any color outside.  The color of the ink must be black.
15.  Fill out the paperwork accurately, completely, and legibly.  If you have questions, ask the officer behind the glass.  if you make mistakes, ask if you can strike through the mistake, or if you need a new form.
16.  After completing all forms, read through the forma again and fill in the signatures, dates and other information you skipped over.
17.  Stand up, and give the forms to the officer behind the glass.
18.  Wait for the officer to review the forms.
19.  Correct the blanks you still left blank even though I told you to check them before handing them in.
20.  After the officer is satisfied the forms are complete, he will tell you when to come back to pick up the form.  Write the dates down when your permit will be ready. 
21.  Take a picture of the dates posted in the window with your cell phone so you'll at least have that after you lose the paper you wrote the dates on.
22.  After the 2 week wait, go to HPD between the hours of 7:45 AM and 3:00 PM, Monday thru Friday, excluding City & County Holidays
23.  Park and lock your vehicle
24.  Make sure you park in a legal space
25.  Make sure you put enough money in the meter for the visit -- usually an hour is plenty, but you can leave and feed the meter if there's a line
26.  Make sure you have your valid Hawaii Driver's license or other valid photo ID
27.  Go to the Firearms section and stand in line.  If there is no line, wait at the doorway to the firearms section to be called.
28.  When called to the window, step forward.
29.  Tell the officer behind the glass you are there to pick up a pistol permit and show him your ID
30.  After you get the permit, return to the same gun shop you bought the receiver from.  You may use the receipt they gave you as reference.
31.  Park at the gun shop, and go inside.
32.  Hand the gun shop employee your permit and valid ID
33.  When handed the DOJ/ATF Form 4473, fill it out according to the instructions given to you by the employee.  Mistakes are not allowed, so if you mess up, you have to start a new form.
34.  Make sure the correct box is checked in box 18.  HANDGUN. 
35.  When all paperwork is complete, the gun shop employee will give you the receiver.
36.  Within 5 days, go to HPD between the hours of 7:45 AM and 3:00 PM, Monday thru Friday, excluding City & County Holidays
37.  Park and lock your vehicle
38.  Make sure you park in a legal space
39.  Make sure you put enough money in the meter for the visit -- usually an hour is plenty, but you can leave and feed the meter if there's a line
40.  Make sure you have your valid Hawaii Driver's license or other valid photo ID and the receiver
41.  Go to the Firearms section and stand in line.  If there is no line, wait at the doorway to the firearms section to be called.
42.  When called to the window, step forward.
43.  Tell the officer behind the glass you are there to register a receiver.
44.  Place the permit, receiver and your ID under the window.  (Note: there is no need to perform the chamber clearing process of a lower receiver without a full upper receiver attached)
45.  When the officer hand you all of your items back, he will also give you a registration form.  Fill it out completely, accurately, and legibly.
46.  Before handing the form back to the officer, review it for completeness and accuracy, because you remember what happened last time you were there!
47.  Sit patiently while the registration is typed.
48.  When called to the window, stand up, take the registration form, and review it.  Make sure it is complete and accurate. 
49.  If you find a mistake, tell the officer.
50.  Once the form is accurate, sign the form.  No need to sign every sheet -- the forms have carbon paper between them to produce duplicate copies automatically.  For more information on carbon paper, how it's made, and how it works, please see this reference:  http://en.wikipedia.org/wiki/Carbon_paper
51.  After you sign the registration form, remove the top copy by tearing along the top perforated line.  Return the rest of the form copies and carbon paper to the officer behind the glass.
52.  Thank the officers behind the glass for being patient with you, as I'm sure by now they have gone above and beyond their normal duty to help you.
53.  You now have a legally registered AR lower that can be built as a handgun.
54.  Go straight to your home and remove the lower from your vehicle before you get caught transporting a "firearm" to a location not authorized in HRS 134.
55.  Review the other threads on here on their experiences building AR pistols. 
56.  As soon as the receiver is completed, take it back to HPD between the hours of 7:45 AM and 3:00 PM, Monday thru Friday, excluding City & County Holidays
57.  Park and lock your vehicle
58.  Make sure you park in a legal space
59.  Make sure you put enough money in the meter for the visit -- usually an hour is plenty, but you can leave and feed the meter if there's a line
60.  Make sure you have your valid Hawaii Driver's license or other valid photo ID and the receiver
61.  Go to the Firearms section and stand in line.  If there is no line, wait at the doorway to the firearms section to be called.
62.  When called to the window, step forward.
63.  Tell the officer behind the glass you are there to re-register an AR receiver as a completed AR Pistol.
64.  Give the officer the completed firearm, your valid ID, and the registration you received from them for the receiver.
65.  If everything is satisfactory, they will hand you your items back. 
66.  If they give you another registration application, fill it out accurately, completely and legibly.  If they simply make ink changes to the old registration, that's all you need to do.  Skip the next step.
67.  If you have to complete another application for registration, wait for the new registration to be typed, review it when called to the window, sign the top copy, and retain the top copy while returning the other carbon copies to the officer.

If you have any further questions, please contact the firearms division at HPD and let them know you are not competent to own firearms.  No other steps are required after that!   

For numbers 1 and 31, do I need to park in a legal space?  I only ask because you specified that I should do so in 7, 24, 38 and 58. 
Also, for numbers 1 and 31, does it matter when I go to the gun shop?  Will they not mind if I enter when they are closed?  I only ask because you specified days and times for visiting HPD.

Honestly I haven't figured that out yet. 5.56 is certainly the most prevelant but there are at least a half dozen different types of ammo available to shoot on a AR platform. Could shoot 9mm, .300 blackout, .308, 762x39 ammo, etc. Not sure how these types of ammo would perform a short barrel though so I think I have some research to do. 5.56 was never my favorite home defense round especially in something you cannot rapid fire. I would look towards a larger round. What are your thoughts?

Since pump action only comes in 556 guess your options are limited.
I'm saving to build a 7.62 upper to hunt deer and pigs.

This kit seems like it would work for any caliber available.
http://www.bentwoodgunsmithing.com/#!/cuy0

The aesthetics aren't the best and it doesn't look like it would fit a real short barrel, maybe 8 inch would work? That might be modifiable though by trimming the pump handle. The concept itself doesn't seem that complicated, basically a handle to push the rod back but mounting a sliding handle a little more difficult to engineer.

Don't know if you realize this, but if you just remove the gas tube on a Direct Impingement AR15 and plug the hole in the gas block or barrel, you now have a manual firearm that will cycle only by pulling the charging handle.

If you're trying to build a pistol, that might be the best configuration.  Then your barrel length isn't predicated on the amount of room the pump action fore end requires. 

Rate of fire will be slower than pump action which, according to rational, common sense gun safety proponents, makes any gun much safer.

Just a thought.

Suka pointed that out earlier but really isn't what I was thinking of.

I play paintball and I ended up using only pump guns to play now. They are just more satisfying. More skill to use well but simpler, less to go wrong. You are more involved in the manipulation of the gun. Maybe that is why I am leaning towards a pump design. Even the pump rifle would be cool to have.


What caliber/bullet would you go with?

Suka pointed that out earlier but really isn't what I was thinking of.

I play paintball and I ended up using only pump guns to play now. They are just more satisfying. More skill to use well but simpler, less to go wrong. You are more involved in the manipulation of the gun. Maybe that is why I am leaning towards a pump design. Even the pump rifle would be cool to have.


What caliber/bullet would you go with?

Caliber is determined by the intended use and length of barrel.  If only for target, I'd use 5.56/.223.    I wouldn't use one for anything my life might depend on, like for hunting or defense.  That's like jumping out of a plane with a hone made chute!

So again I ask what caliber would you go with?

I am leaning towards the 300 blackout as it is large (200 grains IIRC) and supposed to perform out of short barrels well but has limited range. Then there are those two new bullets in the 6mm range that I cannot remembe rtheir name right now. I think all of these rounds are somewhat pricey though. Certainly the .308, 300 blackout and the 7.62x39 would be more useful in pistol hunting.

I answered the question.

So what round would you use if not for target shooting?

Mauidog, saw in your instructions that you said the registered pistol receiver had to be re-registered as a pistol once it was completely.   Are you certain of this?  Since it is already registered as  a pistol, I don't think you have to.   In this HHAA article, the attorney notes there is no reregistration requirement if a registered firearm is rechambered, re-barreeled, etc...

http://hawaiihistoricarms.com/whats-the-law-modification-of-my-registered-firearm/

Mauidog, saw in your instructions that you said the registered pistol receiver had to be re-registered as a pistol once it was completely.   Are you certain of this?  Since it is already registered as  a pistol, I don't think you have to.   In this HHAA article, the attorney notes there is no reregistration requirement if a registered firearm is rechambered, re-barreeled, etc...

http://hawaiihistoricarms.com/whats-the-law-modification-of-my-registered-firearm/