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Acronyms:
CLTA = Commercial Leaded Training Ammo
CFRHT = Center Fire Reduced Hazard Training
JHP = Jacketed Hollow Point
FLETC = Federal Law Enforcement Training Center
ICE = Immigration & Customs Enforcement
SSA = Social Security Administration
NOAA = National Oceanic and Atmospheric Administration
CBP = Customs and Border Protection

http://www.huffingtonpost.com/2013/02/14/homeland-security-bullets_n_2688402.html sums up to 1,400,176,000 rounds max over 5 years. The 750M number comes from a DHS FLETC spokesperson, but the actual documentation for the 750M solicitation says otherwise.

The majority of the 1.4B number is comprised of the SCEMS-11-R-00004 450M (.40 JHP) and HSFLGL-12-B-00003 750M solicitations. Reading the documentation for the 750M buy, it's actually (70M*5years)=350M, and if you keep reading (HSFLGL-12-B-00003-000002.pdf), that number is amended to 70M TOTAL for the 5 year contract. So alternative media reports 750M rounds, but the solicitation is for 70M rounds. 70M rounds of CLTA ammo of various calibers.

HSCEMS-11-R-00004 is 450M rounds (over 5 years) of .40 Smith & Wesson Caliber 180 grain JHP, duty grade ammo.

Solicitation #: HSCESS-09-Q-00003 (.40 S&W Ammunition (135 grain))
Office: ICE
Dec 19, 2008
Awarded Date: August 17, 2009
Award Amount: $38,227,600 (Federal Cartridge Company)
https://www.fbo.gov/index?s=opportunity&mode=form&id=f57aeb05452eb7aa778531ad2b3aaa00&tab=core&_cview=1
https://www.fbo.gov/index?s=opportunity&mode=form&tab=core&id=eea6de5420ac8d01e5b7cb953559eb9a
200,000,000 Rounds JHP (over 5 years)

Note: I am looking for the award to Winchester for a very similar order (200M .40 cal JHP) http://www.winchester.com/library/news/Pages/Winchester-Awarded-Contract.aspx

Solicitation #: HSCESS-11-R-00001 (9mm and/or .38 Special Ammunition) (no award notice found)
Office: ICE
May 9, 2011
https://www.fbo.gov/?s=opportunity&mode=form&id=d2fb936b24b27ffdce0eacc9208516d8&tab=core&_cview=1
25,750,000 Rounds JHP? (over 5 years)

Solicitation #: HSCEMS-11-R-00004 (.40 Smith & Wesson Caliber 180 grain JHP)
Office: ICE
July 12, 2011
https://www.fbo.gov/?s=opportunity&mode=form&id=feed76f5e1253db8938c7756f621d4fc&tab=core&_cview=1
http://www.prnewswire.com/news-releases/atk-secures-40-caliber-ammunition-contract-with-department-of-homeland-security-us...
450,000,000 Rounds JHP (over 5 years)

Solicitation #: HSCEMS-12-R-00002 (.223 Caliber Remington Enhance Performance and Standard Duty Ammunition)
Office: ICE
February 6th, 2012
https://www.fbo.gov/?s=opportunity&mode=form&id=140e263e1c4b2654e61adf022688eb5d&tab=core&_cview=1
175,000,000 Rounds (over 5 years)

Note this is the solicitation that news outlets got the 750M number from, and if you read the document, it says: "The maximum for the resultant award(s) is 70 million rounds per year" and it's a 5 year contract. So the actual maximum number is 350M. Then the solicitation was amended on Aug 16, 2012 to "The maximum limit for the resultant award(s) is 70 million rounds for the life of the five-year contract(s)"
SCOPE: This specification applies to handgun, shotgun, and rifle ammunition strictly for training purposes. This type of ammunition is designated as commercial leaded training ammunition (CLTA).
Solicitation #: HSFLGL-12-B-00003 (Commerical Leaded Training Ammunition)
Office: FLETC
April 17, 2012
https://www.fbo.gov/index?s=opportunity&mode=form&tab=core&id=d024b4476b2648e5c1c7856f3e7cf5e3&_cview=0
70,000,000 Rounds CLTA (over 5 years)

Solicitation #: SSA-RFQ-12-1851 (Request for Quote for Ammunition) (not an award notice)
August 7, 2012
Office: SSA
https://www.fbo.gov/index?s=opportunity&mode=form&id=6c39a2a9f00a10187a1432388a3301e5&tab=core&_cview=0&fb_source=message
.357 Sig 125grain JHP Quantity:174 Unit:TH
174,000 Rounds JHP

Solicitation #: DG-1330-12-RQ-1028 (Ammunition and Shooting Targets) (award notice not found)
August 9, 2012
Office: NOAA (Note: http://www.businessinsider.com/this-is-why-the-noaa-supposedly-ordered-46000-rounds-of-ammunition-2012-8)
https://www.fbo.gov/index?s=opportunity&mode=form&id=bfd95987a1ad9a6dfb22bca4a19150cb&tab=core&tabmode=list&=
16,000 rounds .40 S&W caliber, 180-grain jacketed hollow point (JHP).
24,000 rounds .40 S&W caliber, 180-grain jacketed hollow point (JHP).
6,000 rounds of frangible, 125-grain CFRHT .40 caliber.
46,000 Rounds

Solicitation #: HSCEMS-12-R-00016 (223 Duty Ammunition)
August 14, 2012
Office: ICE
Award Amount: $4,643,040.00
Award Date: July 30, 2012
https://www.fbo.gov/index?s=opportunity&mode=form&tab=core&id=311eb3ee003671285de8db1036b2b255
assuming the rounds are $0.20/round that is 23,215,200 Rounds
ESTIMATE: At $0.20 per round that is 23,215,200 Rounds
23,215,200 Rounds (Estimate)

Solicitation #: HSCEMS-12-R-00015 (.223 Remington Caliber SD 62 grain) (not an award notice)
August 16, 2012
Office: ICE
https://www.fbo.gov/index?s=opportunity&mode=form&id=c432be5826bf182b180ab4c2392aa978&tab=core&_cview=1
15,000,000 Rounds

Solicitation #: 20072346-JTC (40 S&W training Ammunition)
Sept 19, 2012
Office: CBP
https://www.fbo.gov/?s=opportunity&mode=form&id=35e9d23f7b8315916ba65de7e13d18c4&tab=core&_cview=0
120,000,000 (training) (?)

Solicitation #: 192112NFU0000345 (9mm Luger +P Caliber 124 grain JHP Duty)
October 18, 2012
Office: ICE
Awarded: September 24, 2012
Award Amount: $4,870,008.50 (Ammunition Accessories, Inc. DUNS# 009066481)
https://www.fbo.gov/index?s=opportunity&mode=form&tab=core&id=e3b539af526bb5b39534eb4dd3af5e0d
Estimate: At $0.20 per round, that is 24,350,040 rounds
24,350,040 rounds JHP (Estimate)

Solicitation #: HSFLCS-13-Q-00009 (40 Cal, 180 Grain, JHP)
December 17, 2012
Office: FLETC
Awarded: January 2nd 2013
Award Amount: $45,758.00
https://www.fbo.gov/index?s=opportunity&mode=form&tab=core&id=14886846a8fc85aa5939b7014acc3593&_cview=0
40 CALIBER, 180 GRAIN, (JHP), 1000 ROUNDS PER CASE, 200 CA
200,000 Rounds JHP

Solicitation #: HSFLAR-13-Q-00014 (Ammunition)
February 5th, 2013
Office: FLETC
Awarded Feb 21 2013
Award Amount: $38,559.60
https://www.fbo.gov/?s=opportunity&mode=form&id=7e01b30fd254914d565c21064aaddc16&tab=core&_cview=0
CLTA .40 caliber 165 Grain, (JHP) 100,000 ROUNDS PRICE PER 1000 ROUNDS 100MX
CLTA 9MM 115 Grain (JHP) 100,000 ROUNDS PRICE PER 1000 ROUNDS 100MX
CLTA 9MM BALL 124 GRAIN 40,000 ROUNDS PRICE PER 1000 ROUNDS 40MX
240,000 Rounds CLTA

Solicitation #: HSFLAR-13-Q-00020 (CLTA .40 caliber 165 grain JHP)
Mar 21, 2013
Office: FLETC
https://www.fbo.gov/index?s=opportunity&mode=form&id=22419062d923f87db4df15c676da7943&tab=core&_cview=0
CLTA .40 caliber 165 grain, jacketed hollow point (JHP) p/n P40HSTS3G
360,000 Rounds CLTA

UPDATE: Janet Napolitano: "DHS typically uses 150 to 160 million ammunition rounds a year"
http://washingtonexaminer.com/article/2527529

The Framers Didn't Want National Police Force: http://www.stephenhalbrook.com/articles/framers.html

http://www.infowars.com/us-senator-big-sis-buying-ammo-to-dry-up-supply/